There is also the fact that (I know some people disagree) it is important to not arbitrarly just have every type having reliable wide distributed stab.
Part of the allure of competitive pokemon is the fact that types are different. If every type had a accurate 120 BP phisical stab (with recoil or -def/spdef) and a 80 BP accurate special + 130 BP with -2spdef move, then we would just have a rock paper scissor with no other shenenigans.
This is even more important for strong offensive types like rock or flying.
Flying type is specifically designed with the idea that they don't have an accurate special stab unless they want to commit to rain, and their phisical stab is only distributed to birds and bird-like pokemon.
i doubt the balance concerns because, well, people have always said the lack of strong ghost moves (physical and special) is also intentional, and now there are a bunch of overtuned ghost-type signatures moves, and also poltergeist, a move clearly suited for competitive use over in-game use.
Emphasis: *signature* moves.
Signature moves are important because they are that, unique to the pokemon. Ogerpon cornerstone has had usage for example *specifically* because of its accurate 100 BP rock stab. If every rock type had a 100 BP rock stab, noone would have ever considered it.
Calyrex Shadow would not have had the massive impact it had if it wasn't for Astral Barrage and had to just use Shadow Ball (note: AB is a spread move, extremely relevant for VGC).
Distributing strong "unique" special moves is fine. Poltergeist has, unsurprisingly, been kept away from certain pokemon like Dragapult intentionally, because their intent is that "if you want to use this mon to its highest potential, you have to commit the gimmick to it, you don't get a free out of jail card".
There is a major impact difference in "giving one pokemon a reliable strong move" and "giving 300 pokemon a reliable strong move".
So no, I don't see a issue with Gyarados, Dragonite and few others flying type not having access to a flying stab that doesn't suck or require a gimmick (note: Dragonite actually has Dual Wingbeat which is a pretty strong move regardless, 90% accuracy is a fine price to pay for the benefit of disregarding focus sash).