Something that I never saw in discussions regarding Riot vs Valve in terms of how they handle esports is
their income source.
Valve's income source comes from Steam, not CS or Dota, there wasn't any pressure for Valve to implement franchise system compared to Riot.
Riot earns money through Skins and other in game purchases, simply put League and Valorant ( to a lesser extent ) are what keeping them as big as they are. Knowing this it's easy to see why Riot implemented franchise system.
That said, I hate franchise system. I will explain why in my next thread