Yes. Generic rule-sets tend to be less popular. This is why with GURPS, the setting books in total sold much more than the rules books (before 4th ed when they essentially gave up on setting books and tried to thrill the gearheads). With any rpg, the question players always ask is, "But what do we do?" Clashes over what should be done are a leading cause of game disappointment, for example if you try to gun your way through things in playing Call of Cthulhu, or try to have a lengthy and detailed backstory and character concept in AD&D1e. The game has one answer and you are trying to force through another.
With a generic game, the question "but what do we do?" is answered with "anything you want!" But if you can do anything you end up doing nothing.
I say this as someone who has written and sold generic rpgs. But I think mainly of my own game table, where I can supply the setting. Writing it up would be much harder. Rules are easy to write, settings are hard.
You wrote, "But if you can do anything you end up doing nothing." However, it doesn't sound like you have that problem yourself (or have you?) I and the people I have played with generally haven't had that problem. Because as you say, our GMs provide settings and circumstances, and our players also tend to be creative and interested enough to come up with things to do without needing to be artificially pollinated with adventure seeds or dragged around by adventure hooks.
Of course there are many players who aren't like that, especially at first, and for them there should be good ready-to-play materials. It seems to me though that such could be provided for a generic system - they just often aren't.
I've always thought GURPS made a marketing mistake by not providing more ready-to-play content early on. The second GURPS title, Orcslayer, was great because it was ready for immediate action. It had full battle maps and combat stats for everything, complete travel rules - everything for several days of gaming with near-zero GM prep required. Then they did almost nothing like that again for years and years, IIRC. The settings books (with a few exceptions) were also just starting points for developing settings - they weren't developed settings - they mainly talked about what the setting would be like, providing some minimal mechanics for assorted things, a few sample characters, etc. They were like "here's the start of your research on how to create a setting in this style"... or at least that's how it seemed to me compared to the level of prep I do. I could I suppose half-ass improv a campaign using one of those worldbooks, but really I'd want to do tons more world building before I'd run a game, of which the worldbook would only give me a start on the research. Still would be really useful and/or interesting, except I pretty much never want to game in most of the worldbook settings they published anyway. Didn't stop me from buying nearly my own weight in GURPS worldbooks, but I was usually disappointed because I was hoping for more crunch - rules and stats for stuff I could incorporate into my own worlds.