In the past the UCI has banned bikes from competition, for any number of reasons. Most of it was about keeping bikes all the same, so anyone that broke a record did so on the same style of bike from years ago.

But now they let racers use carbon fiber bike that are far more aero than say a 1920 round tube bike. Why is this so???

I mean, your subject line is true, but admittedly your reasoning is not what I figured you would cite. Of the 50 reasons why the UCI could be called inconsistent, I really never would have put 'shaped carbon fiber tubes that arent like 100 year ago' on the list.
Go you for complaining about the UCI in a different way than most.

Mfg could make plain round tube CF more or less one inch in diameter, They would be of the same dimensions as a 1920 bike that someone set a record on, and not have today CF bike aero advantage.

One of the primary drivers of UCI regulations is that regular consumers have access to products used for racing on the open market, promoting the so-called trickle-down effect. To that end, they have been successful, and racers use equipment that can be purchased off the shelf at any top-end bike shop. I am unsure how this is controversial, but somehow, I have a bad feeling about this, and the conflict is about to begin.

Because your premise is faulty.

Their race; their rules.

I blame Henri Desgrange who broke down and allowed derailleurs in the Tour de France in 1937. It's been downhilll ever since with all this new fangled stuff.
Brent

Round metal tube frames were used in UCI races up until about 2000 (aluminum), and some aluminum bikes had aero-shaped frame tubes (Trek's "hydroformed" tubing comes to mind) but a lot of other features on bicycles have changed since the 1920s, too. You can still buy a metal-framed bicycle but its not going to have the cutting-edge components on it you'll find in more modern bicycles. For comparison, have you ever tried to drive a 1920s car versus a 2020 car? It's a world of difference. If today's cars were made like those of the 1920s very few folks would be driving them.

UCI is backed by French royalty, has been corrupt from its inception

The races UCI controls are supposed to be more about the cyclist's abilities than the machine they ride. So for one thing, it's important to control what those machines are and prevent any one bike from having such a unique design that might influence the outcome of a race more so than the strategy and strength of the cyclist.

Bikes and equipment must also be what is available or what in the very near future will be available to any consumer.

Your post title shows a little ignorance IMO.

It is very possible to build a carbon fibre bike that has the appearance of a round tube steel bike. However the only tube on this one that is completely round is the head tube

It's ok, the UCI knows that the real aero gains are in the socks.

How can the UCI even allow the various tires being used... They all should be using tires that anyone can get at the local Walmart.

Why does the OP even care about “racer boys”? On the bright side, at least it’s not another “I’m doing it righ and you’re not.” thread. Nice to see someone expand their repertoire for a change.

Tires are marginal gains. However they do have rules for tires too.

I guess Robespierre should have finished the job.

Yeah I can never understand that part. They want to ensure the bicycles are production bicycles that regular people can buy, but somehow the tires are allowed to be special one-off 'prototype' tires.

No, the UCI is controlled by the Romanovs.

I think the UCI should allow recumbents.

Because it's not 1920 anymore ...

I think prototypes are allowed, and they're often available to the general public (if you have the scratch) later that year or next year. E.g. 12 speed Dura Ace, VIttoria Corsa Pro tires, etc.

Waiting on E assist bicycles to be UCI legal... going for that podium!

Not the Hapsburgs?

Doing a tour stage or a full tour on vintage or modern vintage replica bikes would be cool but not practical and not going to happen.

In terms of the UCI they are actually owned by a cabal of lizard people who operate out of the non-existent basement of a pizza place in D.C.