Because the UNIX-based security model is 50 years old.
UNIX underlies most widespread OSs, and even the big exception Windows has been influenced by it more than is apparent. It stems from a time when computers were big, expensive, slow machines exclusively used by arcane specialists.
At that time, users simply didn't have extensive personal data collections on any computer, not their university server, not their personal computer (and certainly not their mobile phone). The data that varied from user to user were typically input and output data of scientific computing processes - losing them might constitute a loss, but largely one that could be compensated by re-computing them, certainly nothing like the consequences of today's data leaks.
Nobody would have had their diary, banking information or nude pictures on a computer, so protecting them from malicious access wasn't something that had a high priority - in fact, most undergraduates in the 70s would probably have been thrilled if others showed an interest in their research data. Therefore, preventing data loss was considered the top priority in computer security, and that is adequately ensured by regular back-ups rather than access control.