This can be done, it’s called nuclear fusion and the sun does it constantly. The practical limitation is, largely, overcoming the electrostatic force.
Basically, because the atoms are similarly charged, squeezing them together is like trying to push together the North sides of two magnets: they repel each other. It takes a massive amount of energy to squeeze them together hard enough to overcome that repulsion, fortunately the sun has enough energy to do this fairly easily. It is much more difficult for humans to do.