There's definitely a whole lot of overpriced garbage audio products being sold. The reasons are varied, but I think a lot of it boils down to the fact that you hear what you want to hear, and what you want to hear is that your new $3000 amplifier sounds better than your old $300 one.
That being said, if we take the LA90 as an example, it's really lacking a whole lot of features that you could get in a similarly priced integrated amplifier, like a remote control, streaming, phono input, and headphone output. Is it worth it to trade those features for extra SINAD that you may or may not be able to hear? Depends on what you're looking for, right? Not saying that you shouldn't buy the LA90, far from it, just that there are things other than performance to consider.
That being said, if we take the LA90 as an example, it's really lacking a whole lot of features that you could get in a similarly priced integrated amplifier, like a remote control, streaming, phono input, and headphone output. Is it worth it to trade those features for extra SINAD that you may or may not be able to hear? Depends on what you're looking for, right? Not saying that you shouldn't buy the LA90, far from it, just that there are things other than performance to consider.