It's a limitation of the crysis engine, it maps a point where it thinks a person should be sitting in a vehicle (in a mech, the cockpit), and sets a radius around it where it will allow you to enter. Since the cockpit's a good ways off the ground, it forces you to have to jump up to the torso of the mech to get in.
You'll get used to it after awhile, and it's great jumpjet practice for your BA skills in the meantime. If it really gets to you from time to time, switch to using tanks temporarily so that you don't have to deal with it.
And welcome to the forums!