So part of it is that it took a long time for instrumental music to achieve the kind of status that warranted designating pieces as these autonomous individual things. Before the late eighteen century, what a Symphony largely was was "the thing heard at the beginning of an opera before the story begins" or something like that. So like, they were "functional" music and they got "functional" names. These names helped communicate to people what they were for, which helped them understand what the music was doing and why.

Keep in mind that there was very little sense at this time that music had the capacity to communicate something by itself (partially because our sense of what it means for music to communicate something is a bit different, but maybe that's another conversation). So the most illuminating, descriptive, and useful thing to call a piece was by its genre name. If I tell you I've written "Sparks in A major" then who knows what the hell is going on in that piece! But if I say "String Quartet in A major," then you can get quite a lot of information from that: starting with the ensemble that plays it, but also in the eighteenth century information about what kinds of movements you could expect in what order, and even the kinds of events you could expect to happen within one movement. So genre tells you a ton, it in fact told eighteenth-century audiences pretty much every thing they needed to know about a piece.

Things change somewhat in the nineteenth century and you start having colorful and "programmatic" titles. This is where my "Sparks in A major" piece might fit in more. But also, by this point to write a piece called "SYMPHONY IN A MAJOR!" meant that you were doing something prestigious, that you were producing something that can be and in fact should be judged as though it were one of the "great symphonies" by previous masters. Now a name carried not really a set of listening expectations connected to genre (though there's still some of that, definitely), but rather carries a sense of history about it. When you want to be a part of that history, you use the name that invokes said history.