It's a major hassle and if you are going to get a small box to plug in all kinds of other small box around it with a web of cables, you might as well get a bigger box and put it all inside...
This doesn’t seem to be true, and I don’t even get what it would change if it were true. Developers aren’t the target demographic of the base version with low storage.
Docker in macOS (at least the useful one) just runs in a Linux VM, and I don't see why you couldn't run a VM off an image on an external drive. Maybe the UI doesn't let you select that location?