Because good enough is ... enough. Because of the learning curve Because when you have to use another computer you will suffer.
I know that everywhere I go, no matter what I use (computer, phone, tablet, kiosk, etc) it will have the same keyboard layout.
If you try to improve it, then either my computer is better and everyone else is stuck on the same old keyboard (little benefit) or there are numerous keyboard layouts I’d have I deal with (deficit for everyone) until a new king is crowned.
We seem to have (unintentionally) decided as a society that what we have is good enough and there isn’t a keyboard good enough to make the pain of switching layouts worth it.
Look into the history of the keyboard and the typewriter. Commercial typewriters have been around since the 1870s, and the Dvorak layout was created in 1936. Why didn't that layout catch on?
Look into alternate keyboards that exist for computers. Look up keyboards like the Maltron, Kinesis, the OrbiTouch, and others. Why didn't they catch on?
I'll give you one hint for the answer: The sunk cost fallacy.
Good luck!