Fox's Up Smash actually has head invincibility not foot invincibility. It's been this way since Melee. Why not head and upper body or lower body invincibility? I don't know. Aerials are strong in Smash. Anti-airs pretty much suck unless you have an actual anti-air like Mario and Ryu or use a weapon like Marth or Shulk.
Smash's invincibility placements at times are really stupid. It's better in recent games, but there are points where you'd go, "Huh, maybe Ganondorf having leg invincibility for Up Smash would be good", except it doesn't. In Captain Falcon's case, instead of invincibility, it has a ******* disjoint. This also goes for other properties like transcendent priority where I don't think any normal projectiles should be transcendent, so not stuff like Bowser Fire Breath, Fox Blaster, Greninja's charged, somewhat slow moving Water Shuriken, or Zelda Din's Fire, but stuff like Falco Blaster shouldn't have been transcendent. Or something like transcendent priority on a non-disjoint such as Falco's Side Smash and Zelda's Ftilt? isn't really useful. If anything, it's just incentive to not make those moves good since it would be pretty stupid if they were good and had transcendent priority.