It wouldn't surprise me if the original reason is the pervasive use of jupyter notebooks in ML, which don't adhere to normal python conventions, and are affected by slow imports only when those sections are explicitly evaluated.

Side-effects in imports are, in my opinion, unnecessary, losing some of the benefits of static analysis, running with different parameters during tests, compiling to native code (if those tools exist), slowing things down, and more.

Libraries could have an initializer function and the problem would go away.