It's not clear you understand what tenure is.So I heard long ago about some BS about professors needing tenure because 5000 years ago someone got mad about some research and someone got fired and blah blah blah.
Enter the real world.
Why does tenure still exist at universities?
First, "tenure" means different things at different universities- tenure may include salary support or not, it may include laboratory space or not.
Second, universities may indeed fire tenured faculty for other reasons: specifically, "financial exigency".
Third, award of tenure only means that faculty cannot get fired without a hearing before a faculty committee.
Institutions are free to *not* have tenure- and there are several (Cleveland Clinic is one; also the Keck Graduate Institute)