Oddly enough the human body also hasn't changed much. I've noticed this in some other technologies as well, such as the electric bass guitar, which I play. The electric bass settled on a fairly standardized geometry within a few years of it being invented. And while there have been some experiments and oddball instruments made over the years, the bread and butter instrument still looks pretty much like the original Fender bass with some tweaks. I've tried some of the modernizations, and they're physically awkward to play. There was a period of basses with sexy long necks and tiny little bodies, and they all suffered from "neck dive."

Still, while I ride a bike that would not have looked out of place in 1890 when viewed from a distance, on closer inspection a lot of things have been improved. Perhaps most importantly, all of the newer materials are better, including high performance steel, aluminum, tires, and so forth. Hydroforming and carbon fiber layup have finally made it possible to experiment with more interesting frame shapes.

There are a lot of alternative bike geometries that go a lot faster than the standard bike position. They reduce wind resistance and/or have better biomechanics. The lack of change has more to do with Union Cycliste Internationale rules than with optimization.

Indeed, and in fact I see a lot of recumbents in my locale on the longer, flatter bike paths. Not so much in urban traffic. So a bike that's adapted to the human body doesn't mean there's not a better adaptation for some uses.

And I'm certainly not averse to trying one someday -- N+1 and all that. There's a shop in my locale dedicated to them. However, I think the mass market for bikes has diverged from the racing world. Most bikes sold today are of styles such as cruisers, hybrids, and low tech mountain bikes, that are unrelated to racing categories. Even many racing style bikes such as gravel and cyclo are bought with the intention of using them "off label" if you will, just because they look comfortable and rugged. So the UCI isn't holding us back any more.

Hell, there's the whole category of e-bikes. That's probably the biggest threat to the market for more efficient human powered bikes.

That’s mostly because the UCI has very strict rules about bicycle design. They banned recumbent bikes very early in the history of the sport; more recently they banned Graeme Obree’s “superman” position and other changes to the geometry of a normal upright.

http://www.wolfgang-menn.de/superpos.htm

https://www.bikeradar.com/features/top-five-banned-tech-the-...

Check out Mike Burrows for some interesting bike designs. Check out the Battle Mountain speed records for what the highest performance bikes look like.