There are a couple of legitimate reasons, namely the expense/KYC process of an Apple Developer Program membership and/or the complexity of integrating signing + notarization into existing build pipelines (but XCode does makes it pretty straightforward to cut an ad-hoc release that is signed and notarized).

In my opinion at least, the most likely reason is that Apple is refusing to notarize the software. If this is the case, people really should not be running it.