To start with, it assumes the Wizard both has Fly in her book and has it prepared. I have played many Wizards but only 2 total even had fly in their book. I am playing one of them right now and she does not even have it prepared (she has Summon Shadowspawn, Fear and Counterspell prepared instead).
Just because you didn't, doesn't mean that nobody else doesn't.
And let's be honest: What wizard
doesn't get a spell that basically trivializes any terrain based challenge? Either from getting it by leveling up or by buying/finding a scroll and copying that into their spellbook?
Second a Wizard's flight in many ways is inferior; it is concentration which means just a single hidden archer could make her plummet to her death with a bad con save. Now she could have feather fall prepared too as a failsafe, but now she has two spells in her book and prepared to let her fly to the top of the cliff.
Yeah, there's
always an archer parking themselves around every cliff/gorge that exists just to shoot anybody who casts Fly down.
I wonder how much that job pays.
Finally, a 3rd level slot is a high price to play to accomplish this when it is available on numerous races at 1st level and available to all Druids without using a slot by 8th (and some Druids earlier). Further unlike a Wizard's Fly spell these methods are a lot safer with only a small risk of plummeting.
And then the Druid gets shot by the same archer that's hiding near the cliff. Not only that, but because the bird they're turning into probably doesn't have a lot of health, they are now
guaranteed to be plummeting to their death, rather than having to make an easy save that any spellcaster worth their salt is gonna have things to help them pass that save, like high Constitution, Proficiency in Constitution saves from Resilient, and/or just straight up advantage on concentration saves from War Caster.
And on choosing a race that gets a flying speed, you must not be aware of the occasional kerfuffles that happen whenever a player chooses to be an Aaracokra, the only race that gets a flight speed at 1st level (that I'm aware of, at least). And when it comes to the other races that can
get a flight speed, tell me the average level that they get that flight speed, because I can guarantee you it's at or around the same level that spellcasters can start casting Fly.