Because upper registers take time to develop
Let's say you have two teenage boys. One of them has a range from Ab2 to D4, and the other has a range from C3 to F4. What are they? They're baritones! At least that's the only repertoire either of them will be able to sing. For tenors and sopranos, a lot of what defines their repertoire lives in the upper reaches of their registers. That second kid is probably a tenor, but until he gets his A4, he's never going to be singing those songs, and his high school music teacher, who was almost definitely not a voice major, will tell him he's a baritone.
It's not that there are more adult tenors than baritones. If you go to any given untrained group of adult men, you're going to be labeling almost all of them baritones, and indeed, I think most men are baritones if only because it's the "middle" voice part. But some of those guys will actually be tenors, and their lack of high notes isn't due to lack of natural talent, but because they don't have the breath support and throat relaxation necessary to hit them. You might think there are more adult tenors just because tenors tend to be shoved front and center more often than baritones, but that's a casting and cultural thing, not a true representation of statistical frequency.
Hope that shed some light on your question. Thanks for the a2a. Hope u like it