There are just so many round things.
A human egg.
A dandelion seed head.
A black hole.
A frog egg.
Hydrogen and Helium atoms.
A rolling stone.
Protons and Neutrons (we think).
Galaxies (some would argue that many are elliptic or spiral, but those are basically
round structures, too).
A cell nucleus.
Saturn, Jupiter, Mars, Venus and Mercury, the planets that can be seen by eye.
A Sand Dollar.
Crop circles (by definition, although other shapes are found too, but there
are very few straight lines).
An avocado seed.
A ball bearing.
The waves that form from a stone thrown into a pond.
A thermonuclear blast.
The light effect from a point source.
A walnut, especially when it’s still on the tree.
Trees (OK, their trunks are long and fractal, but in cross section, round).
A meteor strike zone.
My waistline (ahem).
The spin of the water in the toilet bowl.
Baby food spattered on the floor.
Round, round, round.
From the smallest, to the largest, everything is round. From Protons to Galaxies. All animals start out as being round, in the form of embryos. The forces all create round fields. Circles and spheres (which are just rotated circles) dominate the landscape, including both the living and the base matter. Roundness is a form that pervades each and every aspect of the reality that surrounds us. And so, in order to create a system that can most easily describe the universe that we inhabit, it would be best if we utilized one that was good at describing:
That, by the way, kind of eliminates Cartesian geometry.