Some leases specify that sub letting is not allowed and doing some would put the original tenant in breach of the lease agreement.

I'd even say most leases vs some, especially regarding residential. Buying a house for the purposes of renting requires different financing (although most are being bought for cash) than a typical homeowner's financing which has clauses that says the person receiving the financing will use the address as their primary residence.

These things get ridiculously specific.

Subletting isn't an agency relationship. It's not like tenants provide a service to landlords by occupying the property as well as paying rent.

Also, the law around real property including leases is different and more involved than straightforward contractual relationships.