Why do you have to be buried after dying from a vampire bite to become a vampire spawn?

Question

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?

Answer 1

Vampires are "chained to the grave"

As clarified in the earlier descriptive text at the start of the Monster Manual entry on vampires (p. 295), vampires and vampire spawn are bound to their sites of burial or entombment:

Chained to the Grave. Every vampire remains bound to its coffin, crypt, or grave site, where it must rest by day. If a vampire didn’t receive a formal burial, it must lie beneath a foot of earth at the place of its transition to undeath. A vampire can move its place of burial by transporting its coffin or a significant amount of grave dirt to another location. Some vampires set up multiple resting places this way.

The vampire's grave site and burial are metaphysically significant to their nature - a vampire cannot exist without a grave of some kind that acts as its resting place, from which it ventures at night and returns to during the day. A creature killed by a vampire's bite who does not receive a deliberate burial will not become a vampire spawn unless "naturally" buried somehow - such as by a landslide or cave-in. Either way, they must be in conditions that could be described as a grave or tomb in order to raise as a vampire.

The "Stake to the Heart" weakness that vampires and vampire spawn have specifically references their resting place:

Vampire Weaknesses. The vampire has the following flaws:

[...]

• Stake to the Heart. If a piercing weapon made of wood is driven into the vampire's heart while the vampire is incapacitated in its resting place, the vampire is paralyzed until the stake is removed.

And the vampire spawn version:

• Stake to the Heart. The vampire is destroyed if a piercing weapon made of wood is driven into its heart while it is incapacitated in its resting place.

Vampire spawn, though less powerful than "true" vampires and slaved to their master's will, are still vampires with normal vampiric strengths and weaknesses, and they still require a grave of some kind to rest in.