Well I imagine it’s because it doesn’t fill up your internal drive for an optional extra. I have mine internal but on iMac so having permanently attached fast SSD for more storage isn’t a problem.
I don't know what happened between 2016 and now. Before that, I just installed Mac OS and Windows on the same internal drive. I don't remember exactly what happened but I seem to recall that some people installed Windows to an external drive since they had difficult installing Windows internally due to the T2 chip. This does not seem to make sense since Apple provides Bootcamp drivers.