Hyperscaler companies or the TOP500 don't need high reliability, especially the latter.

With cloud computing reliability is achieved through software, distributed software which needs to be horizontal.

On a mainframe reliability is achieved through hardware (at least as fast as user software is concerned), and the software is vertical.

If you need to run vertical, single-system image software, the cloud is useless for making it reliable.

Systems built on the cloud are reliable only insofar as people can write reliable distributed systems which assume components will fail. It is not reliable if you can't, or don't want to write software like that (which carries a significant engineering cost).

The real reason to avoid mainframes (and VMS) is vendor lock-in, not technological.