Question

Why does it take so much heat and pressure to create helium and other elements in stars?

Answer 1

You need to bring two hydrogen nuclei close enough for the strong nuclear force to bind them, but you are working against their very powerful mutual electromagnetic repulsion (they're both positively charged). At very high temperatures, they are moving fast enough that when they collide, they have enough force to push agaisnt their mutual repulsion until they get very close, and the strong force (which has a very short range) can take over. You don't need high pressure to get fusion per se -- you can get fusion in a near vacuum, easily. But if you don't have high pressure, you get very little fusion, because you have very few nuclei per unit volume to collide.