Question

Why can't the Earth heat up to allow hydrogen fusion reactions like the Sun?

Answer 1

Earth can't produce fusion reaction, due to it's small mass. The Sun and other stars, due to their gravity ( matter = mass = gravity force) inside, within their volume, change the balance between Coulomb's, Strong, Weak forces and cause a fusion reaction. It is not a simple process. Lot of conditions lead to a fusion reaction. Thus it is not simple to create a fusion reactor. Consider the fact, that for a nuclear reaction we have a ready fuel in the nature, produces from a fusion reaction. If we try to achieve the "parent" process, to produce matter (fusion), we shell create in our "Lab" conditions, similar in the stars.

Answer 2

earth isn't big enough