Little value vs cost. Basement floor is generally far enough underground that temperature not vary much with surface air temperature. The ground is an insulator. If going all out everywhere, than can do basement floor as well, but compared to value of adding more insulating value to roof, walls and windows/doors, you get much less bang for the buck.

Note that this does not apply if building slab on grade or over a crawl space. In those cases, floor insulation is important.

One typically uses rigid foam (XPS) in 4x8 sheets under the slab. In our area (MA) r-11 under the slab is required in areas that have adopted the stretch code. We did twice that much (4"). It's cheap and makes a huge difference in the comfort of the floor, both warmer in winter and eliminating condensation and humidity in the summer. Spray foam seems possible I guess, but harder to install and more expensive.

The insulation value of dirt, especially wet dirt, is approaching zero, no idea where the idea that "the ground is an insulator" might have come from.

It's also really hard to retrofit insulation under the slab if you change your mind later.

Well, the ground does insulate from the colder air above ground and prevents air movement; but it's probably more accurate to say that the ground is a huge heat sink/mass and doesn't vary much over the year, not that it has high insulation value. I agree with you that insulating below improves comfort year-round. I think the key question is whether the basement is going to ever be fully utilised as living space, and if so, better to do it - as you very rightly point out, hard to retrofit later.