Why are recumbent bicycles and velomobiles illegal in UCI bicycle racing? 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.