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.

Some peripherals require configuration after connection. Waiting for this can suck. I'm including monitors here -- a bad implementation will present, as an example, monitors out of order. Another problem you run into is inadvertently cycling through connections too quickly, which can show as devices being disconnected during initialization, causing some systems to flag the device as malfunctioning and refuse to initialize it next connection.

Of course it's easy to say that these are silly problems that should have pure software fixes, but Microsoft and Apple don't exactly accept patches. Let's not pretend that software is optimal.

You can see for yourself by disconnecting your monitor in a multi monitor setup or opening your laptop lid if you use the laptop on a dock. Computer windowing systems seem to take a moment to reposition all the windows and might not remember to redistribute your windows when the monitor count changes.

For many years I was happy with my multi-monitor setups. I'd use multiple screens for work, and turn all off except the center one for watching movies or playing games (because the backlight bleeding of the other screens was annoying, even with a 100 % black image).

That's not possible any more. Microsoft, VESA, nVidia say no. This mustn't be possible. (Actually it used to be possible to add a registry key in Windows 7 that disabled all of this bullshit, but Microsoft wisely decided in Windows 10 to remove that. It must not be done.)

Turning compliant DisplayPort devices off while using a compliant operating system must always garble all windows and shuffle desktop icons around. The spec says so. So it must be. The will of the ancients.

(nVidia will allow you to turn this off, if you are using a Quadro card - then you can use EDID emulation even on display-ports, which causes the OS to think the display is always connected - and more importantly - turned on. One of these areas where Linux is ahead -- although Linux is back to garbling windows like the rest in case you do actually decide, as an individual with agency, not a compliant operating system, to change the arrangement of displays.)