How did people come to believe that history and technology progress in "one straight line"?

This is a very difficult question to answer, because the idea of technology progressing like the tech tree of a videogame is a bit hard to pin down. It's one of those big picture questions that runs against the highly specialised nature of academia, but I'll give it a go.

What do historians call it?

What you are describing is a component of a wider historiographical view called 'Whig' or 'Whiggish' history. It was a popular 18th and 19th century view that sees history as progressing toward something. It is still hugely influential, especially given the tendency to teach history as a sort of march toward the present day, as if there is something inevitable about the societies we live in or as if the present is the end of history. Whig history sees humanity as on path to ever greater enlightenment in every sense; personal liberty, political representation, economic prosperity, and technology. It tends to go hand in hand with the idea that some eras and historical figures (ie the supposed "Dark Ages" of the Middle Ages) were holding us back from this progression. Applied to technological progression, the notion that the invention of the steam engine was one step on the road to AI is thoroughly Whiggish. "Whig history of science" is probably the label that comes closest to what you're describing, or maybe "historical progressivism".

Where did that idea come from?

The Whigs were a loose political group in the UK that encompassed many influential historians (like William Stubbs) and scientists (like Charles Darwin), but they're not where Whig history originates. A basic notion that events well in the past indicate the direction of the future goes back a long way, but for the kind of 'inevitable march toward progress' view we can skip straight to Georg Wilhelm Friedrich Hegel and his book The Phenomenology of Spirit, published in the 1800s.

Hagel was one of those philosophers who tried to write a theory of everything, but the small part of his writing that this question cares about is his theory of geist. Geist meant a lot of things to Hagel, but basically he uses it to describe an ethereal cultural force that moved human history and human society. Not a literal force or ethereal being - it's not God - but the notion that cultural forces and moments change humanity, whether that was a 'spirit of the age' (zeitgeist), a 'spirit of the nation' (wolkgeist), or the combination of those things to guide what he called weltgeist, which means the broad trajectory of humanity. This could happen very slowly as cultural shifts, or rapidly due to major events like the Napoleonic Wars. Hagel famously referred to Napoleon as a personification of weltgeist; a man who did not merely live in geist but who made it his own through great deeds. Eventually, Hagel thought, weltgeist would reach a level of self-awareness where 'the geist knowing itself as geist' achieves absolute knowledge and enlightenment, and that progress would reach its end point where nothing further could be done. Hagel is somewhat convoluted btw so I hope I've managed to make at least little part of it understandable.

Now, history doesn't stop. Hagel was wrong to think that greater cultural consciousness would result in a rapid march toward perfection (I mean, we're more culturally connected than ever thanks to social media, but I don't think anyone thinks of Facebook, Twitter, and Reddit as the harbingers of human perfection). But The Phenomenology of Spirit was read widely by the intelligentsia of Europe, who found its idea of geist intriguing and persuasive. They also liked the idea that humanity was on a path to bettering itself on a gradual march toward perfection, an idea called Progressivism.

Progressivism was particularly influential among Whigs; one of the two major political groups in early modern Britain. The other was the Tories, but Tory views on history and technology weren't actually that different to the Whigs - they both found things to like in Hagel. The prevalence of Hagel's ideas about human progress was so great among the Whigs that we tend to call a progressive view of humanity 'Whiggish', though Whigs specifically thought of history as progressing toward constitutional government and personal liberties (and a bunch of other stuff, but that's tangential). Hagelian philosophy and Whiggish history weren't without their ardent critics - David Hume gave it a good kicking in his The History of England - but it definitely took hold; became the geist, if you will. The idea that all things human, be it politics, tech, or scientific knowledge, were on a road to enlightenment was entrenched. The glorification of things that brought us 'forward' became common, and the condemnation of things that 'held us back' did too. My own encounters with Whiggish history come from the idea of the Middle Ages as essentially that point in the road where humanity got a flat tyre and waited for the mechanic to get them going again via the Renaissance. The Whiggish view of history managed to transcend the Whigs themselves, and historical progressivism was largely shared by their political rivals the Tories, and even by the likes of Marx and Engels, it's just that these groups disagreed on what the end point of progressivism should look like.

What 'progress' actually means is difficult. Eugenicists thought their ideas were progressive in that they thought they were making humans as a species better. The temperance movement thought it was causing progress by banning alcohol because drunk men were a burden on society. These are ideas of progress that have not aged well. Banning alcohol in America didn't make things better, it made Al Capone rich (I'm simplifying of course). Eugenics didn't improve the human species, it brought out the worst in us through the systematised murder and forced sterilisation of groups deemed undesirable by the governments of the world. In terms of things like political policies or social programmes etc. what actually constitutes 'progress' is deeply contentious and complicated. Just look at how the word 'progress' is used in current political discourse, and how vitriolic that discourse can be.

However, technology needn't be so inconvenient or controversial. Eugenics as progress is extremely problematic, but the idea of gradually making better modes of transportation until we all have our own little spaceship isn't. As you say, that's not inevitable, and if sci-fi of the 20th century has taught us anything it's that we can be very optimistic and inaccurate about technological progress by our own time. I'm still waiting on those hoverboards and flying cars! Now, there is a lot of talk among historians, scientists, and historians of science, about viewing tech and science in a Whiggish way. There has been since the 60s. But in terms of the general population, who don't have to think about it as part of their job, it's quite easy to take the idea of progress, look at the development of technology, and go "yeah those line up". I think - and this is necessarily speculative but I hope given what I've written you can understand why I think it - we view technology in this way because we find the notion of 'progress' appealing, are basically stuck with it, and technology is an area of human endeavour where it isn't blatantly contentious.

Sources:

Alvargonzález, David. "Is the history of science essentially Whiggish?." History of Science 51.1 (2013): 85-99.

Butterfield, Herbert. The Whig interpretation of history. No. 318. WW Norton & Company, 1965.

Jardine, Nick. "Whigs and stories: Herbert Butterfield and the historiography of science." History of science 41.2 (2003): 125-140.

Knights, Mark. "The Tory interpretation of history in the rage of parties." Huntington Library Quarterly 68.1-2 (2005): 353-373.

Stocking Jr, George W. "On the limits of ‘presentism’ and ‘historicism’ in the historiography of the behavioral sciences." History of science (1965): 211-218.