It's almost entirely about screen resolutions. In the Turbo Pascal era the screen probably had 640x480 pixels or 307,200 pixels in total. The screen I'm looking at now is 5120x2880 = ~14.7M pixels.

If an app is maximized it has to render ~48x more pixels per frame, probably with anti-aliased text which older systems didn't use. Doing that on the CPU is not only visibly slow in many cases, but a waste of CPU time that could be better used for actual app logic. OpenGL happens to be a convenient and portable way to get the GPU to do work for you, at the cost of some memory, so GL it is.