I think the bike industry is killing themselves off due to the push for new and better stuff. And then companies not continuing to support the old stuff.
It isn't that bicycles are good for 5 years and then done. There are quite a few 10, 20, 30, 40, and even 50 year old bikes on the road today.
Then, even if one limits oneself to a half dozen brands, there is a huge number of parts.
So, the bike shops are choosing not to carry $100K or more worth of small parts inventory, tools, & etc.
And, certainly they don't want to get stuck with dead stock that must be deeply discounted to move.
So, they get hit by a customer like the OP, that wants to push the limits a bit. New hydraulic brakes. (Well, not that new, they've been out for several years). A cassette that isn't "stock".
I'm not quite sure how auto parts stores do it, but their entire model is to have stock for just about anything one drives. And, quick turn over for anything they don't have on the shelf (ok, so they're feeling the push of the internet too).
But, really, bikes should be a lot simpler than cars. But, the shops just need to keep a decent inventory.