It’s almost like flight breaks a lot of things in 5e RAW.

By level 15, your expected to be somewhat broken. As someone who has never DMed a game without someone picking a flying race, I get it.

Fly speed is outright better than normal movement speed. There is nothing you cannot do with fly speed that you can’t do with normal movement. It negates a lot of pre written adventurers, almost completely.

Flight is more work for the DM. The player never has to “account” for flight while they play. They can just walk. But if the DM doesn’t account for flight, one character suddenly overcomes problems without rolling a dice. They beat maze puzzles without thinking and without expending resources. They can kite melee opponents forever, just following along throwing ranged attacks as the enemy flees after realizing they don’t have any ranged attacks. This is free exp, when 80% of the monsters in the game are melee focused. The other 20% are spellcasters.

So for a new DM, they let the player get to a “reasonable” level before granting the DM more work to accommodate that player. Giving them time to understand how 5e works and hopefully learning they need to Homebrew a lot of they want to be challenging after level 8. By level 8 the average party can nova down a Tarresque in under a combat minute. 600HP isn’t that much as soon as people overcome resistances, which most do by level 5-8.

TL;DR you aren’t meant to fly until your basically a god. That’s what 5e is balanced around. Think I’m joking? Give your rogue a broom of flying one time and watch them never get hit again as they sharpshooter snipe from 400ft in the air with a heavy crossbow. Give your wizard a carpet of flying and watch them cheese any climbing/moving check ever made. Athletics? Nah carpet. Acrobatics? Nope gonna fly. It’s just more work for your DM as they have to design every single battle map in 3D.