I think they're so bouncy because there's no reason to make it a complicated Jbeam, nor is it a good idea to make them break apart. Engines will rarely come out of the compartment, so it doesn't really matter.
Because when in reality ground deform under impact with engine, in BeamNG ground is pretty solid, impact energy can't just disappear, so stuff generally likes to bounce more than IRL. Also IRL upon impacts quite some of the energy is turned to heat, which does not happen in BeamNG, deformations are cold in game, adding heating upon deformation might help, but probably is not going to happen because huge added complexity. So sometimes structures are tweaked to account that lack of heat aspect, but with engines there could be some limits of what can be done while keeping engine from destroying itself. Also it can be that bounciness of engines is not tuned to absolute perfection, maybe increasing some damping might help, or then it would just break things.
Thanks for the replies ! Does Newton's third law go in here? As for every action there is an equal opposite reaction. So something like I push the wall with the strenght of 3 newtons, the wall pushes me back with the strenght of 3 newtons.
The trick would be throwing an engine made of a solid material on a ground surface made of the same solid material.