Exactly, most texts that were "lost" predate the printing press. Things get messy when you're dealing with manuscripts whose proliferation rely on an individual's willingness to copy them (and ability to copy them correctly.

What will happen most of the time is that other writers will reference a particular work, but it just hasn't been passed down to us, or it's been passed down in an incomplete form, or it hasn't been catalogued properly very very early on so no one knows where surviving copies are. For example, in my field (Northern Italy) there's a text called De Magnalibus Urbis Mediolani, an invaluable description of life in thirteenth-century Milan which was referenced by Antipope Alexander V some sixty years after it was written, and more importantly, was cited by the near-contemporary chroniclers Heinrich von Herford in Germany and Galvano Fiamma in Italy. Sounds like a great primary source, right? One problem: no one's seen a copy since the 15th century, the Visconti (the rulers of Milan) mention it in a library catalogue in 1426, and then nothing.

In 1894 the Italian philologer Francesco Novati was doing some research on early Milanese writings and travelled to the Biblioteca Nacional de España, which housed important Milanese manuscripts taken to Spain during the period in which the Duchy of Milan was integrated into the Spanish Crown. Among the Lombard manuscripts, he discovered a copy of De Magnalibus Urbis Mediolani. Now, this manuscript (catalogue number 8828) is far from perfect; the copy was transcribed around 1380 by someone named Gervasio Corio who committed multiple grammar errors and to top that off the parchment was not stored properly and suffered humidity damage. Today, it has been withdrawn from consultation, however Novati published his personal transcription of the manuscript and this is the text consulted today. Using this text, we can re-construct what we think the original was like based on the author's style in his other writings. So although we know this work existed and we have a fairly good idea of what it says, it would appear that the original text just hasn't been passed down to us; it's been "lost."

Even in the contemporary period there's a lot of documentary material that we generate that is just...ephemeral. H. P. Lovecraft (1890-1937) is a good example: along with his fiction work, he's been cited as one of the great epistolarians of the early 20th century, with dozens of correspondents in all walks of life on all manner of subjects, with correspondence that varied from a line on a postcard to 30, 50, even 100 handwritten pages in one instance. Many of Lovecraft's letters were kept by the senders, some were lost or destroyed - what you might expect, for someone that wasn't famous during their own lifetime! After Lovecraft's death, his friends and correspondents August Derleth and Donald Wandrei founded a small press called Arkham House to publish Lovecraft's work, and one of their big projects was the publication of his letters - a project that took decades to complete, and only resulted in the five-volume Selected Letters of H. P. Lovecraft, which contains the highly abridged contents of some 930 letters (it is estimated 10,000 letters survive, and the publication of the unabridged letters is still ongoing). The way Arkham House worked is that they solicited the letters from Lovecraft's correspondents, transcribed them in part, and then returned them - where, in at least a few cases, the owners destroyed them. So the Arkham House Transcripts, with the gaping holes and all, are the only record we have for thousands of pages of Lovecraft's letters.

It really just boils down to the fact that a lot of historical documents aren't seen as historical or worth saving at the time they're written.

As the other writers, in particular /u/AncientHistory, have noted, it's hard to know what will be historically valuable at the time it's created. Most people don't think of themselves as historically important, so they have no systematic method for saving their documents. When they move, or their accountant tells them they can toss them, or when they die, large numbers of documents just get tossed. Even for famous people, people in government, or other notables, there can be lots of compelling reasons to destroy documents, including privacy (and its cousin secrecy), storage room, or just plain old neglect. In fact, neglect and accident have always been drivers of the loss of historical materials. Especially in the nineteenth century and earlier, when fires were terribly common, materials frequently burned. Floods, insects, wars - these all have led to the losses of materials.

In the digital age we face new challenges. It may seem that born-digital materials (Word documents, web pages, stuff like that) are great candidates for preservation, but the nature of digital technologies means that there is an almost constant threat from technological obsolescence. If you're older than around 30 or 35, it's very possible that you've got a big stack of ZIP Discs somewhere full of old documents. Can you read them? I don't even own a computer with a CD drive, personally. Maintaining old technologies is expensive and time-consuming, and that's before even consider the software ecosystems necessary. Even if you can find a working 5.25" floppy disc drive, you also have to have the correct software, etc. It's a nightmare, and it might get worse.