The certs are sold by certificate authority companies, and Microsoft doesn't get a share of that, though I'm not sure.

Yeah, software being signed says nothing about it not being malicious or insecure, but it does prove the author is what it says, and if it is malicious then the responsible party is clearly visible.

For non-commercial hobby/open-source software the certificate price is prohibitive, so the only 2 options are Microsoft Store or accepting that users will see the scary warnings, and of course complain to the developer about it.