Because if you use a FMJ, the base is open. Open bases are notoriously inconsistent and that degrades accuracy. Even Lapua discontinued their FMJ match bullets.
They are HP because everyone wants the utmost in length consistency for seating at the most accurate OAL. Soft points have inconsistent lengths and the tips are easily damaged.
Yes, the empty space/balance in the jacket has an effect on accuracy, however small, with Berger introducing their "column target bullets" that is supposed to have that empty space optimized.
Tyrone mentions the reason in hi first paragraph.
The way bullets are made is that a ram pushes on a copper plate to make a cup that is then filled with lead. You can make the open end of the cup, the bullet jacket, either at the front or the back of the bullet. The most important part of a bullet as far as stability is concerned is the last little bit that touches the rifling as it exits the muzzle, that is, the base of the bullet. In order for that to be the most uniform and consistent, manufacturers fill the lead in from the other end - the hollow point.
At least that's the way the US Army Marksmanship Unit explained it to me.
