Non-famous pianists play better than famous ones very frequently.
Then there is the problem of the halls. The biggest names tend to play in the biggest spaces, which are disadvantageous to a large portion of the piano repertoire. Also, it often happens that the qualities that make a particular artist sound special on recordings are simply not audible in Stern-sized caverns. In general I've had better luck with small halls, churches, classrooms, living rooms, etc.
Of course, when I buy an expensive ticket to see an internationally famous pianist in a giant hall, I am priming myself to have a big experience. Expectations are high, and I listen differently, I'm sure. Generally, this probably enhances my enjoyment, but it also makes it harder to be caught off-guard, to be surprised or stunned.
I agree that if you sit in the back of huge halls, you're not likely to get a very satisfying sound experience in solo piano recitals. I'm fortunate in that even in the biggest hall I go to in London, I get to sit quite close to the piano, in a cheap seat (usually <$12). In some smaller halls, I have to sit much further away, and pay more for the privilege....
But over the years, I've pruned down the number of recitals by well-known names that I go to, because some of them are somewhat predictable and play the same repertoire again and again with no change in interpretation. I'll only go if their program contains something I really want to hear, preferably something I've not heard them play before.
I'm more likely to go to one by someone I've never heard live before - frequently a recent competition winner, or someone who's just come to the fore recently because they've just signed a big recording contract (apparently there are still recording companies prepared to gamble on doing that), or promoted by someone famous like Argerich. In such instances, it's the pianist rather than the program that's the attraction. These pianists can sometimes be more interesting than most of the star names, but I often wish they'd be more adventurous in their choice of repertoire. But it's still the case that IMO, the majority of these (usually young) pianists sound much like anyone else, with little individuality or magic that makes them stand out from the crowd. Some sound like they've played the same pieces once too often.
Which is why, when someone like Benjamin Grosvenor comes along (who not just plays with a personal stamp, but also programs adventurous repertoire), it's like a breath of fresh air. But he certainly isn't an unknown, not by a long stretch. I'm still looking forward to the day when I go to a concert by someone not so young with no big reputation, and wonder why he/she still hasn't been taken up by a big agent, been given a recording contract etc, etc.
Until then, I've just have to believe that cream really will always rise to the top, one way or another, and that there is no hitherto undiscovered beverage where the cream has been languishing for some time, still waiting for the opportunity to rise......