Because not every class can have everything. It's as simple as that. But why any Mercenary would be complaining about not having something is beyond me (considering EVERY other class is begging for us to be nerfed big time). Tracers, the ability to heal, a jetpack, flamethrowers, heatseekers - Merc is the bane of most PVP fights and one of the most fiered opponents in the game when played well. And even for PVE, we're given Mako who heals better than most players.
I'm juuuuuust fine with our abilites.