I think a lot of it is down to people not looking into their purchases more if they’re unhappy with the price of things.
* A lot of bike parts are generic parts that you can get cheaply from a motor-factors / local parts shop.
* A lot of specific bike parts can be got cheaply via ebay, bike discount / rose, lidl / aldi.
* How ‘premium’ you want your purchase to be? Do you really need an XT cassette or will a Deore one do?
* Look into alternatives; e.g. don’t spend £12 on 240ml of float fluid but £5 on 500ml of 75W-90 synthetic gear oil instead.
Obviously there are some parts that you have no option to get the manufacturers part at full RRP (maxles and cleats are the only things I can think of for now). There are also parts specific to your particular bike that are genuine rip-offs, but how often do you need to replace these?
If you insist on spending £200 on a new goretex jacket and moan about it afterwards then you’re an idiot. If you’re happy to spend £200 on a new goretex jacket then good for you – I wouldn’t as (a) I can’t afford it (b) if I could I would rather spend £200 on something else (c) would rip it in a week anyway.
Although generally speaking MTB’ing is overpriced, no, bike parts aren’t.