You would be right if housing location (mostly) didn't matter. But people generally want housing close to where they work and "location" is not a fungible input.
To use an extreme example, if every landlord in Manhattan was using Real Page where would you build your competing housing? Manhattan is already pretty densely developed and housing in the other boroughs involves a much longer commute for someone working in Manhattan.
Edit: Another point is that, unlike something like a web service, you can't collect all the customers. You could only steal as many customers as your buildings can hold. Meanwhile the landlords using Real Page are collecting higher rents and can squeeze you out of building additional property by bidding up the cost of the land you would have to build on.
To use an extreme example, if every landlord in Manhattan was using Real Page where would you build your competing housing? Manhattan is already pretty densely developed and housing in the other boroughs involves a much longer commute for someone working in Manhattan.
Edit: Another point is that, unlike something like a web service, you can't collect all the customers. You could only steal as many customers as your buildings can hold. Meanwhile the landlords using Real Page are collecting higher rents and can squeeze you out of building additional property by bidding up the cost of the land you would have to build on.