Because peck is a move that they learn early on. Since pidgey line already learn gust early on, he doesn't learn peck as initial moves, his initial flying move is gust, on the other hand, his rival kanto bird Pokemon spearow learn peck and does't learn gust for same reason.
Mostly gameplay elements, the game happens as it should in a game, the characters in games usualy have powers, moves, and alike who serve his purposes but not what they actualy don't need or is not made for them for gameplay purposes. For example, in real world a warrior could use any weapon he wants, but in most RPG games he uses the weapon that serve what he has to do, for example, if he is a class of warrior that uses war hammers, he will have war hammers and not swords, even if in real life a warrior could just use swords whenever he wanted.
Same goes for Hurricane, Bravebird and sky attack
Pidgeott doens't learn Bravebird by standarth (only as egg move) because he can learn Hurricane, since his spl attack is almost the same as his attack stat and he especialize in wind moves like gust, his mega evolution give strenght to this fact.
Swanna can learn both Hurricane and Bravebird, because he doesn't especialize in the same thing as Pidgeot for example, he especiallize in versality, having both attack and spl attack stats at the same number and also having the water type to increase his versality. So in the end, he can learn Both.
Swellow and staraptor especialize in Flying contact attacks, for this reason, both can learn bravebird
Also since starly is a bit more powerfull in stats than tailow, Tailow can learn a flying move inicially, "peck", and starly not, to make them a little "equal" in start, since tailow need peck inicially and starly not, but both learn wing attack early on, on level nine.
Unfezant in the other hand are something beetween pidgeot and swellow, he especialize in a mix beetween contact and wind flying moves, for this reason he can't learn bravebird, but can learn sky attack who fits better his style and is a looks alike move. (Besides, super luck and scope lens make unfezant too deadly already without bravebird, if he had, he would be too much overpowered in the hands of the people who knows how to make proper use of it, sky attack give the option he needs but also balance that.)