Why do molecules jiggle about instead of being super rigid?

What are you talking about?
Planty of people have adequately responded to OP's very simple question.
It's just that energy is kind of vague because there are so many forms of it and because so many different things can happen to the molecule because of it.

>why do molecules jiggle about instead of being super rigid?
Because temperature of any given molecule causes the thing to have 'thermal motion'. Do you think you can just apply energy to a molecule and have it do nothing?

the simplest way to explain this to you would to be approximate the energy of the system by solving the shrodel-dinger equation. from memory its something like hookes law

E = h*(n+1/2)*v

The most basic explenation is this. The system must have positive energy (E) and as you can see for this to be true frequency (v) can not equal zero. Hence the system must always vibrate like your mums hot pocket insert

I had the wrong equation but the same thing applies for the actual one, that one up there deals with quantum vibrational number which I believe can equal zero hahaha