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

Source: https://tosdr.org/en/service/274

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.