From what I see there are already various extensions on JED (https://extensions.joomla.org) which offer that functionality. So there is no reason to add it to core itself.
Or did you try those and they don't work?

Hi, easier to use the built-in library of Joomla which has several API who can do a great job writing for various applications. If create a single plug-in for registration and login, then multiple plugins will be easily written. It only needs to the user's linked and, if desired, unlinked it. The method of authentication itself is very easy.

Not every potential feature needs to be included in the core of your selected CMS. Features aren't "better" because they are included in core, features aren't more secure because they are included in core. The core package is really nothing more than an opinionated set of features, and even with those a bulk of people don't use them.

I'm closing this feature request since such 3rd party plugins already exist and thus the feature is already available.