Ares actually build a BattleMech for Desert wars.
(which was quickly pointed out to be completely useless in modern combat, but hey, it looks cool.)
Otherwise the problem with mech suits would be, overpowered in game , completely unreasonable for a runner to have(way to expensive and state-of-the-art Military equipment), and any Runner who somehow got one would end up with a massive manhunt against him. It would be like using a tank or something, if you run around with that, corps are gonna take you seriously, which is bad because they have an entire army and you don't.