The content providers have limited control as well. It's not like they can put on commercials not paid for (well, they CAN meaning it is possible, but that would be stupid).
They could boycott ad server companies that replay the same ads over and over. There MUST be others out there with a larger variety.
I don't use the Roku MLB app anymore, but their issue used to be that they'd show the same MLB promos over and over (or had a rotating set of 4-5). But with all the breaks in baseball, they got very old, very fast. Are they showing actual commercials now?