One thing that comes to mind is that as a developer you don't have to pollute your main partition with a beta if you can install it on an external drive.
AFS has containers to install betas on the same partition, the main partition is not polluted (and the additional volume(s) can be removed at will). cfr. https://support.apple.com/en-us/HT208891
I'm not saying that it's "normal" not to be able to boot from external devices, I'm just answering your point
This is quite risky - previous beta updates (such as Big Sur) did break installing updates on older OS releases in the same APFS container, and some beta updates have in the past done incompatible APFS updates that broke the ability to boot old OSes.
