Why hasn't Ethernet gotten any faster in the last 15 years?

Because most consumer PCs have no earthly use for gigabit bandwidth, much less even higher, and the existing Internet infrastructure is nowhere near ready to go beyond, having
spent twenty years getting this far.

I just upgraded my home network. I am contracted to get 10Mbps from my ISP, but with my brand new docsys 3.0 cable modem and a Netgear Nighthawk 4X 2350 AC wireless router, I can sometimes get 56Mbps over wifi---when that bandwidth is available. That's because my ISP only has do much local bandwidth to share.

When I upgraded, my primary writing workstation (my treadmill desk) went from 5.8Mbps (using an old 2Mhz router as a repeater) to nearly 30Mbps (using a new Amped AC wireless repeater). That's a noticeable difference. Dropbox syncs faster. Pandora loads faster. Youtube is more responsive. Etc.

I cannot, however, tell any difference between my 13Mbps connection out in the yard and the 56Mbps sitting in the bedroom. Why? Because at those speeds, latency on the backend is a bigger factor than my network. 13Mbps is plenty fast for HD video once the buffer is full.

I have about 20 active network clients, including two kid's laptops, two parent's work computers ( using VPN), two parent's private laptops, two workstations, two HD streaming TV appliances, and a a VOIP phone. We use them all the time, and even on the old network (which peaked occasionally at 21Mbps) we just never had any issues (The TVs use Cat5, so are limited only by the cable modem). All the upgrade has done is address coverage issues and bring the weakest network clients into the fold.

So I'm doing all this, peaking at over 50 Mbps, averaging about 20 throughout the house) and only paying for a minimum of 10. 1 Gbps is equal to 125Mbps. What the heck would I do with it?

I submit that if you need more than 1G ethernet in your home, what you are doing may not be consumer grade, or at least to toward the bleeding edge of consumer grade.