Because the suppliers of IT systems (eg Microsoft, Crowdstrike) do not agree that they can be used for life-critical purposes

If someone is injured or dies because the hospital has inadequate backup processes in the event of a Windows outage, some or maybe all liability for negligence falls on those who designed the hospital that way, not the IT supplier who didn't agree to it.

If your assumptions rest on corporate entities or actual decision makers being held legally liable, then you've got a lot of legwork ahead of you to demonstrate why that's a reasonable presupposition.

Because it's evidently a bad idea and there are reasonable alternatives.

That’s easy for you to say, with the benefit of recency bias, and with presumably zero experience in running a hospital.

That's not about experience, that's about following the regulated standards. This is well known ever since technology (not computers) got into hospitals.

None of the points you mention detracts from the correctness of his/her statement.

And? People and institutions constantly make bad decisions for which there are reasonable alternatives, and that's assuming that the incentives at play for decision makers are aligned with what we would want them to be, which is often not the case. Not that that ends up mattering much except as an explanatory device, because people and institutions constantly pursue bad ideas even seen in terms of their own interests.

It would be like orthopedic surgeons heading down to harbor freight to pick up their saws instead of using medical grade versions.

The tool isn't fit for purpose

Because you should always have a backup.