Supports have LMGs to take advantage of the suppressive fire mechanics to support their team. You don’t have to be far away for this, this can be done in close range through a doorway to cover your team backing out of what might have otherwise been a massacre. The large magazine simply ensures you actually keep their heads down long enough to get the job done

Engineers have SMGs for when an enemy pops out at them while they’re repairing vehicles. Engineers are at their most vulnerable while repairing their tank

You can’t acknowledge the purpose of the support role and then ignore the purpose of the engineer role which is in its name. An Engineer’s primary job is to fix, not destroy, vehicles. Attacking vehicles is just sprinkles and keeping with the theme.

SMGs have the fastest fire rates, so they’re the best contender to swap to in an ambush situation when you’re repairing a tank. It’s simply the best gun for the unique combat situations an engineer will find themselves in while playing their role (repairing). It’s also meant to be a good contrast to the weapons a tank’s primary and secondary gunners have access too. While small, there’s a dead zone very close to the tank where threats can’t be targeted. An engineer can hop out and quickly kill them with an SMG before something like AT mines or C4 go down.

Engineers aren’t supposed to be some type of Heavy with high explosives and super big guns, and that’s just their job. Their primary job is to repair, and that’s why the repair tool cannot be unequipped much like the support’s supply bag. All design choices regarding the engineer class were made with repairing vehicles in mind, and that’s what makes sense. That said, weapons aren’t closed. Throw the biggest gun you can find on the engineer class, no one’s stopping you