The Gotthard tunnel is also in Europe and some kilometers longer than the Channel tunnel. I think that one has security checks and the other not has nothing to do with one being under water and the other under rock. I'd be very surprised if their security measures will not harmonize in the long run.
Fires in the Channel Tunnel could weaken the tunnel and cause it to flood catastrophically. This came closer to happening than the engineers expected with an accident on a train carrying hazardous goods.
The security checks are fairly minor. They are mostly looking for gas canisters and explosives.
