Because usable cellular Internet is relatively new, the cable/dsl/etc providers should have upgraded their systems literally decades ago and didn’t, funneling federal money intended for that purpose to CEO bonuses instead.

If they are so rural ISPs won’t take them, why would a cell company deploy 5G in a super sparse area?

For example TMO and VZW will only hand out home cell modem if you live within half mile of a major interstate. If not out of luck aside from Starlink.

Well I tried it for home Internet and it was mostly fine but was a little less reliable than wired. I think reliability is important.