Scout's mobility is more accessible and simpler than rocket jumping is, and he also doesnt lose health or DPS by gaining these abilities because they're part of his character. Scout can easily dodge rockets and other non-hitscan projectiles as well with ease, making soldier less effective against a good scout player, despite his low 125 health pool.
Scout does not need a buff. Scout does well in most parts of a map because his weapons do not rely entirely on being in the air, or splash damage, unlike many soldier primaries, which do best in open spaces. Additionally Scout is designed as more of a hit-and-run playstyle and was never intended to be strong.
Soldier and Scout have very different playstyles that dont really counter eachother. A good scout player can, strugglingly, fight a soldier long distance. At close range though, soldier kind of wins thanks to high DPS, more health/survivability, and splash damage. So soldier is a good counter to scout, but does not overpower him.