I was actually making home made hamburger buns the other day and noticed another reason that probably figures into the situation.
Eight is two to the third power. “So?” I hear you cry.
While this is no longer a major consideration, with “batter-type” bread recipes in commercial use, the fact that dividing a single batch of dough by halves, three times, results in eight of the right size buns would be a motivator. Making ten would be far more awkward than 8, 9, or 12. I leave it as an exercise to the student why no one would intentionally set out to make 11.:dubious:
So, historically, there would have been a motivation for bakers to make buns by the eight (easier!) and it gained cultural inertia.