To be honest it probably comes down to being a non trivial problem to get it to be perceptually seamless and a rare enough event that people don't have the incentive to fix it.
Most people aren't plugging/unplugging monitors often
Seems pretty standard in tech offices to a) have one or two external monitors on your desk, and b) disconnect/reconnect them from your Macbook every time you get up for a meeting.
They still handle it horribly though. My favorite is how macOS used to assign the logical displays randomly, so I'd have to fix the arrangement in Displays preferences several times every day.
Most people aren't plugging/unplugging monitors often