> once things have several developers hacking on a project with multiple artifacts and hundreds of thousands or millions of lines of code

That's exactly the case where you need more strictness about passing tests and CI before getting merged.

It amazes me when I come across a major project where the latest committed version often doesn't even build, let alone pass tests.

Yes, there are absolutely cases where you can't test everything; for instance, if you have complex combinatorial configurations and one obscure configuration fails. But that's a far cry from "inevitable in a large enough project". On the contrary, it's more important to avoid in a larger project.