There are three reasons

1) They don't want someone to make profit out of their own property
2) Tenants usually have to undergo a background check. If someone sublets and rents to lets say an arsonist, who's going to take responsibility? The main tenant could just run away
3) When another tenant is added, they have to pay registration and admin fees. That's more money in the pocket that they'd lose.

So basically a landlord or agency needs full control
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