It's more than hidden gems. And lots of bigger games lack quality too.
There's key differences between the indie scene and triple A scene.
A; Marketing. Indie games typically rely heavily on word of mouth vs a multi-million dollar marketing machine.
B; Roles that they fit. Indie games tend to not be "ambitious" in scope. Not saying that I agree with this, but to the average Joe, "another platformer" like Ori would be less "ambitious" than a lot of triple A titles. Again, not saying this is objectively true, nor am I saying that I agree with the thought.
Additionally, indie games tend to wear one hat, but wear it really well, where as other games will try to wear several hats at once.
C; There are a lot of bad indie games, there are a lot of bad triple A games. However, a lot of indie games can be seen with an attitude that would more or less come off as "this game isn't really fun." This issue is seen less with triple A games-- the biggest problem a lot of triple A games seem to have is their "design by committee" philosophy, where several people are trying to define what the vision of the game is. This ultimately leads to an experience where the high moments of it are far less high than any other game that focuses on those moments, but the low moments tend to be a bit higher than other games as well. Hence, them feeling "same-y." This boils down to "pacing is weird," though.
An example of the above would be that Ghost Recon Wildlands is not a game that particularly impresses me. I don't think the game is really a *good* game, nor do I feel it's worth anywhere near $60. But, if I had to choose between it and something like Papers, Please, I'd take GR any day.
D; There are indie games that blow up and become worthy of being "part of a systems catalog," it just isn't very frequent. Minecraft, Stardew Valley, the weird bird VN romance game, Rocket League, the list exists. It may not be long, but it does exist.
Anywho, long story short, there's a lot that goes into it, it can't be boiled down to any one thing, but effectively it's the fact that if you're an average consumer with little-to-no real exposure to video games, triple-A games are going to be (likely) a better value for you than an indie game. It's like comparing pop music to anything remotely obscure.