I expect a few people wonder that and I don't think it's unreasonable to ask. My personal angle is twofold: if we were being paid stuntmen's wages it might negate the need - and it is need - for compensation. The very nature of military injuries, which tend to be somewhat more serious than, say, a strained thumb, often mean the casualty is unlikely to be able to work at the very least in the military, if at all. Normally, if the person is likely to recover, they'll stay in the services, but if it's serious enough to be booted out, they're disabled and unemployed!
I would also argue that, whatever their actual motives, not even front line soldiers join up to be injured. That is the same as Fireman Sam who doesn't join the Fire Service to fall through burning rafters, or Mrs Miggins who didn't go to work in her office to trip over the photocopier lead.
A serviceman's job might involve fighting, but that's completely different to joining up expecting to take a bullet.