Go try to sign into your open-standards-abiding calendar and notes accounts in the Calendar and Contacts bundled with nearly every Android phone on the planet and see how well that goes.
Compared to Android?
Yes.
I have no idea why iPhone users put up with this shit.
It's about 5 years old by now. Not having looked for something in a couple of years and being bummed out by that doesn't make Apple the bad guy.
You have been told that every new OS update brings new features and abilities, and Apple publishes an iPhone User Guide for learning about iOS features like how to change the default mail app (https://support.apple.com/guide/iphone/change-the-default-ap...). It's on you to look.
I suppose it could be possible for apps to prompt the user to change the default, but I’m honestly pretty sick of that behavior on desktop (e.g. I have several browsers installed for various reasons and they nearly all bug me about being default) and would rather not see it copied to mobile OSes too (I don’t believe Android allows apps to show such a prompt either).
EDIT: Actually, there already is a “Default Apps” section right at the top of the page of Settings > Apps. Yeah, if that’s a “labyrinth” then the assumed level of user intelligence is quite low.