Why can't you? That's what VMs are for. And even then, most cross-platform codebases have an abstraction layer that rarely changes. So even testing on one platform can raise your confidence very high.

> Also, it is much harder to enforce "everyone must run each commit through the CI equivalent properly"

Again why? I wouldn't care about the dev's local branch. But what is send to the main repo can be easily scripted to run the CI on every commit. You just send the result back with each commit that fails. They can replicate the same workflow on their local workspace as a pre-push process.