Because it's more expensive to dig (especially when you hit rock after just 4" of soil) and people haven't yet seen the benefit.

I grew up in Texas, and we have a similar thing here in Missouri. It is just too dang hot and humid in the summer. Tapping into the geothermal sink would be great; it just has a big up-front cost to it.

How much would saving $300-400/mo at peak (maybe $1.5-2k/yr) be worth up front? $5k? $10k? $20k? $50k?

It's a matter of ROI for that dig.