Why (according to Piped developer):

YouTube has an extremely invasive privacy policy which relies on using user data in unethical ways.

Here are some things about YouTube:

Tracking via third-party cookies for other purposes without your consent.
YouTube can delete your content if you violate the terms
Reduction of legal period for cause of action
YouTube may use your personal information for marketing purposes
YouTube can view your browser history
YouTube can use your content for all their existing and future services
YouTube gathers information about you through third parties
YouTube can license user content to third parties
YouTube provider makes no warranty regarding uninterrupted, timely, secure or error-free service
Deleted videos are not really deleted
Your data may be processed and stored anywhere in the world
YouTube is only available to users over a certain age
YouTube can suspend your account for several reasons
YouTube has non-exclusive use of your content
The court of law governing the terms is in the US
YouTube collects your IP address for location use

A lot of inspiration came from NewPipe and Invidious.

I created Piped to fix issues in NewPipe and Invidious which are architectural issues and cannot be fixed easily.