House sales are complex enough that most people use common template contracts, simply because it makes the interaction with the required third parties (realtors, notaries, public record offices, lawyers, banks) more efficient by a large enough margin.

Rental however? Almost everyone uses their own crap contracts or some decades old template contract, and usually there is no one involved except the landlord and the renter. And because there is no need for more efficiency, the market can't and won't push for standard contracts.