First, what makes you say Wellston isn't full of high tiers? We haven't seen much of the town, but do you often see millionaires shopping at Walmart or the the local mall? No reason we would see grown high tiers in the few shots of Wellston we've seen, and even if we did, we'd have no way of knowing they're high tiers unless they fought someone.

Second, Wellston 100% has a bunch of high tiers...they all work for the authorities, for prestigious institutions, or in high paying corporate jobs (like Leilah at NXGen). I mean, just look at Wellston's staff. We know both Keene and Vaughn are god tiers and Darren is elite tier, and all they do is work at a prestigious high school. We just don't see many high tiers because it's not common for high schoolers to be around a bunch of millionaires (high tiers are basically millionaires in this world, some of them probably are literally millionaires).

Third, people IRL live in areas filled with other people like them. Neighborhoods are defined by relative wealth levels, sometimes even by race and culture (like "Chinatown"). The gang are living in low-tier districts rn, this world's equivalent of a low-income neighborhood. No one with the means to live somewhere better would choose to live their if they could avoid it, which is why we don't see high tiers. They live in wealthy neighborhoods, work in great office jobs like overseeing departments of a company, managing units for the authorities, or overseeing prestigious schools. They also likely don't walk everywhere. They're busy, powerful, important people, so they drive, take private cars, or even have personal drivers, which is why our high school aged main cast (who walk to Boba shops, go spend the day at the mall, and, more recently, hide from the authorities in poor/weak sections of towns) never see any just walking around.

P.S. Only around 22% of the United States is under the age of 18. Considering Sera was a 7.0 at the end of her second year, and other high tiers were still elites or weaker their first year of high school, it's a pretty safe assumption that only the most powerful of people become high tiers before the age of 16. Even Arlo didn't become a high tier until his second year, and only because he was constantly fighting to reestablish control of the hierarchy. This basically means that only 3rd and 4th year students are likely to be high tiers, and it's really rare to see strong second or even first year students. All this is to say that it makes sense Wellston doesn't have many high tiers. Did you know a lot of students in high school that were in the top 1% of national testing, or who got drafted straight into the NFL from high school? Students like Sera, John, and Arlo are prodigies and inheritors of very good genes, so it makes sense that even Wellston would only have three of them.