I'm not saying egg bomb is stronger, but it makes more sense and isn't terrible. Pidgeot learning boomburst makes no sense. The fact that it would make it stronger doesn't change that. Zygarde would have a more reliable physical Dragon STAB with dragon claw, but since it doesn't make any sense for it to have (What with it being a serpent), no one thinks it should get that.
Aside from swellow, which I've already noted my annoyance with, every user of boomburst is based around sound. Noivern has speaker-like ears, Exploud is basically an organic amplifier/ pipe organ, and chatot is able to copy sounds it's heard almost flawlessly. Pidgeot, meanwhile, is a big Super Saiyan Bird. It throwing eggs makes more sense than it somehow making noise loud enough to hurt other pokemon.
Not only that, but boomburst is mainly only given to pokemon who really need it. It's noivern's only reliable move to hit hordes or multiple doubles/triples opponants, and boost to the otherwise mediocre at best chatot and exploud. Mega pidgeot may not have a terribly deep movepoo, but it can do fine with what it has. Giving it hyper voice would be okay, but boomburst would be too much.