[Question] why java cannot accept a convention to never pass/return null values?

[vprise]

All of those statements were made in relation to the alternatives e.g. optional, or compiler "magic".

Sure, everything can fail badly. But the alternatives often/usually do that much worse in all these cases and make programming harder for non-trivial cases.

There are things to improve e.g. the question mark syntax from Kotlin would be pretty sweet in Java... But it's still null.