Do you think creating a common feature and re-using it is a good idea?
Is this something you would pay for, if such services provided to you?
Obviously, it will save you months of work...
1. I think every project has different requirements, e.g. what if I want phone auth or single sign-on using email. Once you add all those features, your product is likely to become complex like Firebase Auth or Auth0 where understanding third-party APIs is almost equivalent to writing your own system (especially since open-source ecosystem already has key components available). You might argue that the solutions have client libraries to make things super simple, but in real world applications, plug-and-play rarely works to perfection.
2. Projects that are just starting out can't really afford third-party solutions. Once they can, they can also afford to build their own.
3. Solutions to these things already exists, just not in one place. Netlify has a solution for Forms and templates (I think). For Auth, we have Okta, Auth0 and Firebase. For emails, I think Amazon SNS (not sure).