There is plenty of people who lack the skill or ability to do fine repairs. Especially with batteries, if you have no idea what you're doing, it can easily lead to insurances not paying in case you do get injured or damage something.

Because people want a genuine part (reliable battery). I would pay the price if the battery was max 200eur. Nope, Apple is greedy. In my case they wanted 400Eur (together with top case)

So buy a genuine battery then? They aren't hard to find and are under $100.

Wait, it is possible to get a genuine* battery replacement for an Apple device? And no, I'm not being sarcastic, my friend was told by an Apple Store that even an attempt to open the Macbook chassis voids warranty. They refused to replace the battery EVEN WHEN HE WAS READY TO PAY "because there was an unauthorized attempt to open the device". He finally had to go to a repair shop and get it done under an hour (because now the warranty was void anyway).

genuine = guaranteed by Apple to not break your warranty

There is no genuine original battery on the market. Only option to get it is from Apple for 400eur. Every seller is lying about originality.

If it walks like a duck, quacks like a duck, and looks like a duck...well who cares if it calls itself a swan.

Anyone paying $400 for battery is out of their minds. My $100 apparently ‘fake’ battery performs identical as the one Apple sells. Who cares at that point.