That's because dates are extremely complex and subject to all sorts of political and business rules.

Those business rules, daylights savings (datetimes that can never happen, or happen twice), different M/D/Y ordering, different starting day of week, different timezones (including 15 or 30 minute offset ones like India, Iran, parts of Australia, etc), etc quickly complicate it. Oh, and don't forget about making it accessible.

That's before you even get into designers who want the date picker to look the same across platforms instead of using platform specific UI widgets...

I'm pretty sure my version of hell would be implementing a date picker until all stakeholders are happy with it (eternity).