If splat sold, we wouldn't have a 5e. Nor 4e, for that matter.
Adventures sell because the most time consuming part of the game is prepping to run it. DMs continue to be the primary sales demographic.
Splat competes with Splat. Unless you're CONSTANTLY starting over with new characters, you're only going to buy the splatbooks that support your character, and ignore everything else. It's also very time consuming and challenging to get right, design wise, because it has to balance with everything that came before it. And for good-faith's sake, they have to playtest the splat as well now. So they limit the splat, and what splat we end up getting is high quality splat that sells better than anything else, but they can't produce at the scales that they can with adventures.
Adventures also improve the health of the edition at large because they play with what exists, rather than adding more options. They stretch out the lifetime of an edition by giving a themed Season a la what they have with MMORPGs and their annual or seasonal expansions these days.