Ytdyklly

As stated in the title. However, the "superman" position is also illegal, and the question applies to that as well.

Motor racing will often lean on safety to explain why there are so many limitations to how a car can be built and configured. In Formula One, e.g. fairings are illegal - it can't go too fast, as it becomes "too dangerous" (which is of course not defined, but that's a different matter). Other reasons for different types of motor sports may claim that the cars (such as in SuperGT or DTM) cannot be too dissimilar from actual production cars. Sure they are heavily modified, but they aren't purely built specifically for racing from the ground up. However, even these modifications are strictly regulated, e.g. there are limits on practically everything to slow the cars down (anything from the engine output to aerodynamics to tires, active geometry, etc.), again for safety reasons.

Even though these are justifications perhaps not everyone would agree with, they are at least some form of justification.

Now, what excuse does e.g. UCI cycling regulations have for banning recubment bicycles?

It doesn't seem like they can use the safety feature. Would a recumbent bicycle or velomobile really be that dangerous? From what I understand, recumbent bicycles are legal to use in many countries and are sold openly.

What about not adhering to the real world, being too dissimilar from "normal" bicycles? That also seems strange, given that again recumbent bicycles are sold in the first place.

The only thing I can think of is that a recumbent bicycle or velomobile is simply too "weird". People don't appreciate the aesthetics of it. Perhaps they also don't learn to ride such bicycles, and are discouraged from trying them.

Is that the reason why such rules are imposed on e.g. Tour de France?

To a naive and rather inexperienced bicyclist myself, this seems very arbitrary and counterproductive. I would like to have more efficient bicycles in general.

However, again since my knowledge of bicycles is limited I hope there is a host of problems I don't think about that makes recumbent bicycles and velomobiles dangerous and impractical. I'm curious what they are.

Now before someone points out that a recumbent bicycle may have some disadvantages, I'm talking about in general. If a fully faired recumbent bicycle or velomobile has more advantages than it has disadvantages, then why are they illegal in racing?

This seems like a pretty good history of the circumstances around the UCI's banning of the recumbent. Your guess seems to be about right: recumbents were perceived as too weird. It's interesting to note that Faure had been racing on recumbents with success and official imprimatur for a while before they were banned. According to that article, there were safety concerns about fairings even back then.

The UCI has generally gotten more and more restrictive about what qualifies as a "bike" (although they have relaxed some rules after creating them—read the Wikipedia article on the hour record for some insight into the back-and-forth) and their current rationale is basically that a member of the public should be able to look at a racing bike and recognize it as a regular bike. There is certainly a chicken-and-egg aspect to this: if more people rode recumbents (or other configurations of bike), those would look like regular bikes. There is also the argument that achievements in cycling should be more attributable to the cyclist than the bike.

I can speculate about other factors playing into these decisions, but that's all it would be—speculation.

There are a number of factors that come into it. The weird factor is a major one. Cycling is a sport with a lot of history and tradition which many want to preserve. There are many technological innovations which could improve the performance of cyclists, but they each take it one step further away from the roots of the sport. You see examples of this, just look at the bikes of the Boardman era on the track. Bikes with no seat-tube and bizarre frame shapes made for faster times, but really seemed to have nothing in common with the bikes that racers had been riding in these events in previous decades. To get a sense of consistency and maintain tradition in the sport, the UCI brought in the double triangle rule along with a few others to ensure that any innovations that were brought in would still recognisably resemble the bikes ridden in the very first iterations of the tour de France.

As for safety, that would also be a major concern if some riders in the peloton began racing on recumbent bicycles. While a recumbent bicycle might be perfectly safe you ride by yourself or with other recumbent riders, it would be a disaster to mix them in with a peloton of riders on regular bikes. In a peloton, you don't have the luxury of a neat, organised paceline to know where riders around you are. It's a huge mess of riders jammed into a tiny space and to move through it, you take whatever gap you can. You rely on your peripheral vision to know where you can go, because you need to keep your eyes forward to not crash into riders around you. A recumbent being much lower to the ground would have riders moving across thinking that there is a gap only to bump into the recumbent rider. This would lead to far more crashes in the bunch. To allow recumbents in road racing, you would have to entirely phase out regular bicycles in the peloton. The other option is that they could replace time-trial bikes and only be ridden in time-trial stages, as time-trial bikes are also unsafe in a peloton, so are not allowed in road racing, but then there is a second set of rules on which bikes are allowed in time-trial stages.