The vampire's bite attack states that if bitten and the target dies because of the bite and is buried in the ground only then can they turn into a vampire spawn under the control of the vampire:
Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire's control.
But do you really have to be buried? I get that if you get cremated you won't rise as a pile of ash with a thirst for blood, but it seems odd that being buried is a requirement to come back as a vampire spawn.
What is the in-universe (lore) reason why humanoids killed by a vampire's bite must be buried in order to rise as a vampire spawn?