Except that if you read the rules, that's not how the designers present their rules. They don't present combat, skills(other than ability contests), encounters, etc. by talking about two people. Taking the rules as a whole, it's clear that what you are saying here is not what WotC is doing with ability contests. They are saying it requires two, and then providing one contradictory example that CAN involve more than two. They are not perfect.
I think it is more likely that they are saying it requires
more than one and using two as the standard, exemplary reference. It might be accurate to say that the presentation of the rule
assumes two, but nowhere does it say that a skill contest requires exactly two contestants, and no more. That's something you're reading into it where it doesn't belong. Examples in the rules involving more than two contestants aren't a mistake or an accident, they serve to illustrate that the entire rule structure of an ability contest is designed to be flexible in that regard.
Why one Oerth would you think that Crawford & Co. would design a limitation into the ability contest rule that would restrict it to two contestants? That doesn't even make a little bit of sense. For Pelor's sake, it isn't even a discrete system within the rules, it's just an example of how to use ability checks. If you want to do something with an ability, and one or more other characters want to prevent you from doing that or to do it themselves first, everyone makes an ability check and you all see who comes out with the highest number. It is the most basic, simple thing, and you're trying to shoehorn artificial limitations and restrictions onto it on the basis of your assumption that the format of the rule's illustrative text is the rule itself, and sets the immutable structure to which all ability contests must adhere.
D&D doesn't work that way, Max. It moved a bit in that direction with 3.5 and 4e, but that's not how 5e is designed.
If Crawford had intended ability contests to be limited to two contestants, the rules would have said so. It would be explicit. "This is for two characters opposing each other. If you have more than two characters, each acting to oppose the others, use this other rule, as follows: ..." That's not what was done, at all. Instead they established the basic system of doing things with abilities, then refined it with DCs for character vs world situations and "high adjusted roll wins" for opposed checks, or character vs character situations. When there are exceptions, like saving throws, they describe them in detail. The simple fact that there is not an exception set out for resolving opposed skill checks among three or more characters means, definitively, that the basic system for resolving opposed skill checks applies to those scenarios. In other words, since "specific beats general," if you don't have a specific rule for "3 or more," then the general ability contest rules apply to contests among several characters.