Monitor's don't come with gyroscopes, I can't imagine many people who uses rotatable monitors actually rotate the monitor nearly as often as a typical user would rotate a Smartphone, thus the lack of need for one on desktop monitors.
Back in the day there were a few pro monitors that could determine orientation and tell the computer. As multi monitor and higher resolutions became possible, that went away. Places that needed portrait just did it as a second display.
My main monitor is so big I don't need to rotate it even if I could manage a mount to make it possible. When I had 24" screens on arms, I tried portrait once for 5 minutes. Ooh nifty, but for me useless.
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