With actual rent, most of what you're charging for is the location, not the building. That's why rents for flats in London or San Fransisco are astronomical, but those for much larger and nicer houses in Chelmsford or Fresno are a fraction as much. The rentier did nothing to create the value of the location, they just bought a slice of it.

It's not immediately clear that there is an analogue of location for SaaS.

Which is why ytur point about lock-in is interesting. It's not so much that lock-in is part of rent-seeking, so much as that lock-in creates the analogue of "location" needed for rent-seeking (maybe?). Once people are stuck in AWS, Amazon can charge higher prices for shittier services, because people have a reason to be there over and above value for money.