Southlake has always been good. They were just a small town until the late 90s. They won a lot of titles in 2A and 3A in the late 80s and early 90s. Then they spent like 2 years in 4A and got up to 5A with the quickness. They were trying to get Westlake and Colleyville too, but Keller and Grapevine school districts got them. A lot of good D-I players coming out of Colleyville Heritage and Keller Central where those kids go. The rest of the state dodged a bullet on all those rich kids getting together at Southlake

. Euless Trinity gets some of those Colleyville kids too. Colleyville is the only thing between Southlake and Euless Trinity's attendance zones. If Heritage didn't exist, you might get some Tongans ending up on Southlake instead of Heritage

. So that's where you get a lot of the Southlake being dirty accusations. Those attendance zones between them, Colleyville/Grapevine, Euless Trinity, North Richland Hills and Keller Central are all right there together. Could easily slide a kid or two onto your team. But I think Southlake has been pretty clean.
I don't know if I would call Allen rich. West of 75 its all upper middle class. Nice big subdivisions. Houses starting in the 300s. East of 75, they have some regular folks, but then that gives way to some straight up mansions and upper middle middle class houses as you get closer to Parker and Lovejoy. But what they have is that one big ass high school in the whole town. Its different from Southlake because its about twice as big. Like 5,000 kids there. And 95% of them grew up playing together. Plus they get all the athletes in Allen and some from Plano, McKinney and even Frisco that are in their attendance zone. Plano schools are just as big, but they have multiple feeder high schools and junior highs. Most kids didn't play together until 9th grade. Then when they get to high school their schools are only 11th and 12th grade. Allen is the perfect storm that high school coaches want.
I know this because the Allen coaching staff is from my high school coaching staff at Garland. They were open about that being the best job even though Allen was bad at the time.