Here's why:
* Tribute summoning requires a monster to be face-up on the field: Most monsters that require tributes for their summoning require you to tribute face-up monsters from your field.
* Face-down monsters are hidden: Face-down monsters are considered hidden, and their effects and stats are unknown to your opponent. This means you can't target them for specific effects or tribute them for summoning.
Exceptions:
* Some cards may allow you to tribute face-down monsters: There are very rare cards that specifically allow you to tribute face-down monsters as part of their effect. These are usually limited to specific scenarios and not a general rule.
* "Flip Summon" can change a face-down monster to face-up: If a monster is flipped face-up during its turn, it can be targeted for tribute summoning.
In summary: You cannot generally tribute face-down monsters unless a specific card effect allows it.