DPI is one of my pet peeves, together with points. Why not use mm for font size?

Instead of DPI, one could use µm per px, or px per (c)m if you want something easier to work with at very high DPI.

I know what DPI and points are, but just because I know a few values and can say "that one's large", "this one is small", with no idea how much they act iually measure. With other units I could approximate the viewing distance much better.

Those things come with the prevailing manufacturing country, which was the US for printers and such. So people just keep it.