Disadvantage of said system for Google would be the fact that if you do that, people can skip ads much faster and they won’t be able to do any tracking of interaction at all. For advertiser’s point of view, that would be just worse version of TV commercial.
I would also wager that Valve was worried about Microsoft attempting to use “creative” methods to compete with Steam and chipping away at them, like hidden API. Its not like Valve knew that Microsoft’s attempt would continue to flop so hard for decades that they couldn’t even try that.