Hello and welcome back 🙂
While I can’t give an exact answer why the datepicker allows to select past dates on the Community Events submit event form, I imagine it was discussed during the initial planning or a decision made later on.
I did a quick check of our logged/feature reports and don’t see anything resembling what you’ve described, so it doesn’t appear to be an issue for a large amount of users. I also did some research to see if I could find a work around in the mean time but was unable to come up with anything.
Our UserVoice Feature Ideas is the best place to voice your opinion on what you want to see in an upcoming feature and/or maintenance release. Sorry I don’t have a better answer or a solution for you in the mean time and best of luck with the customizations. Cheers!