The simple answer here is that it helps stop people exploiting.
Imagine you and a buddy ran two different loadouts. One of you runs, say, smoke, and the other rocket defense. You start getting locked, you smoke. Smoke fades, get locked again, rocket incoming, you jump out, buddy in, he uses the defense, and you're safe. Around the time the defense timesout, you get in again, and smoke to break locks.
There's also likely the way have cooldowns setup making it a requirement for that vehicle to always be THAT vehicle. If you jump in, the vehicle changes on the tech side, all your cooldowns reset, something like that. Yes, it's possible to set it up so that cooldowns run a unique timer, and any gadget in that slot has to wait on that timer. Likely they didn't set it up like that from the start, and remaking that now would be a headache and a half, even more so with the vibe crew remaining to crank out sub-par content for this game.
