Why summoned unit die after some time?

[WC3Wonderer]

Archmage creates Water elemental from water. Water elemental die after 60 seconds. Mana was spent. Water elemental is dead.
Keeper of the Grove converts trees into Treants. Treants die after 60 seconds. Mana was spent. Trees are destroyed. Treants are dead.
Necromancer creates two skeletons from a corpse. Skeletons die after 65 seconds. Mana was spent. Corpse dissapeared. Skeletons are dead.
Dreadlord calls Infernal. Infernal die after 180 seconds. Mana was spent. Infernal is dead.

I don't understand. Why summoned unit die after some time? Why game cancels my actions?

 

[Bender]

Summoned units are unnatural, their existence is limited by how much mana was spent to create them, thus they cease existing after a while.

EDIT: Forgot to mention previously, but there's also the balancing factor: If summoned units were permanent, players would stall their summoners in a safe place and create literal armies that wouldn't cost gold, lumber or food.

 

[WC3Wonderer]

Make water elemental permanent. Increse mana cost. Remove cooldown. Train six footmans. While training footmans summon six Water elementals Level 1. Send twelve units to attack enemy base. Leave archmage at the base to restore mana.

It is good Archmage. Don't you think?

 

[Bender]

Sounds like you Archmage is now a moving Barracks.

 

[raypack]

Ragebait used to be believable

 

[Tasyen]

Warcraft 3 wants that you use skills at the right time and push the gameplay more into Action-RTS so it is nicer to watch. Summon times help both goals.

It also means that the other player can decide: deal dmg to a summon or just wait it out.