Because it makes life much easier for landlords, tenants, and councils, and makes it easier for poorer tenants to find accommodation that will accept them. Many more landlords are refusing to accept "DSS" because of this horror.
The last Government prevented council paying housing benefits directly to private landlords even if the tenant wanted that. The result is that many landlords are not getting the rent because some tenants are dishonest or just unable to handle money correctly. Worse still, the benefits have to be paid into a bank account and are then snatched by the greedy banks claiming "charges" and "arrears."
This regulation was supposed to be applied to housing associations and councils as well, but they rebelled fearing massive extra arrears and costs.
It is not only landlords that suffer but also tenants who are evicted for arrears. Let the tenants choose if they want to have benefits paid directly to landlords. It gives most of them peace of mind.