Because it makes no sense to design a mouse pointer with unknown range in such a way that it is linearly mapped to range on a viewport.

Users need full range of precision. No one wants to wave their arm around several times to get to the other side of the screen because someone thought it was an excellent idea to make sure every millimeter was mapped to every pixel.

And no one wants a pointer so imprecise that the slightest nudge sends you 400 points left right up or down.

There is no universally agreed upon acceleration curve. So what, do you complain that not every single vehicle has the same foot peddle resistance?