Regular activities are like that too. Locking isn't widely implemented. It's not on purpose just hasn't been implemented.
In the first few years of CiviCase's life it used to always make new copies when you edited an activity. You'd still have no warning if two people edit, but you'd see the previous versions. It wasn't a good technical design performance-wise and in fact you couldn't turn it off. Then a feature called advanced/detailed logging was added that logs changes to all tables in civi, but it's not turned on by default. Again there'd be no warning, but you'd have the history for all records. Then at some point the case activity revisions feature was converted into an optional feature and turned off by default too. Then it stopped working. So, at the moment, as a partial solution you can turn on detailed logging (under Administer - System Settings - Misc, where it's just called Logging). Then at least you'll have the record in the database (it creates another set of tables named log_civicrm_XXX).