Edit: just got a quote for an iphone 7+ battery replacement from one of the mall kiosks here: $120. If there is so much fat in the cost surely they'd at least undercut Apple by $20 to get the business.
Removable batteries are a completely different conversation but you can still buy phones with them so go ahead and indulge yourself.
Undoubtedly genuine Apple batteries may be better quality but that's still ~$100 for labour.
You're not only paying for the labour, but also a guarantee the phone will work as intended after and warranty on the replacement. $100 is cheap for that.
Because it is designed to prevent user from replacing battery, so you pay $100 for opening the lid, taking out the battery, putting in the new one, and closing the lid. $100 is nowhere near cheap for something that should be safe to do for the user.
$100 _is_ cheap for the phone as it currently stands.
Only if you're Apple's fanboy. Virtually anybody else still has replaceable battery.