For one thing, pywikibot also comes with a collection of applications (such as replace.py for quickly replacing text across pages, etc.) as well as helpers (e.g. a series of pagegenerators that make it easy for you to get a list of pages to work on).
For another, myclient is not as actively maintained and developed as pywikibot (which itself is maintained by only a very small group of really interested people). mwclient was last updated in Sep 2021 I think, whereas pywikibot has updates as of this month.
Simply put: mwclient is just a Python SDK wrapped around MW API, but pywikibot is an actual framework.