Getting stuck in the ground/lamppost, etc. probably occurs because an object is pushing you, or you are going into a certain direction, but the collision detection doesn't run every millisecond. So during the time the collision detection isn't running, or isn't running fast enough, it's possible to go through objects.. then you end up getting stuck in them, and collision detection kicks in and says, hey.. you can't go forward or backwards because there is an object you'd collide with.. (the light pole would intersect your car) so you can't do that. Even though it's IN your car already..
I imagine it's something like that. It's more rampant in MTA because the game is running slower usually (more shit on the screen to render), and because there are other cars that can produce a FORCE on you.
Earlier today a bulldozer was driving into me. It ended up driving ontop of me, and I was forced INTO the ground. It's possible to get out again, you ahve to keep trying to move and you can possibly lift yourself up, or someone has to help force you out again.