Steam

I think the reason they don't do that is because the costs of building an ad network that complies with regulatory requirements in every region is really expensive.

For existing ad networks, it's worth it because of the volume of customers. For Valve, the customer count would be low unless they wanted to make a play at becoming an ad network outside of Steam as well, which is not something that makes sense for them as a business. So for Valve, micro microtransactions and other forms of paid DLC are a much better business model for free-to-play games.

I think it's better than having ads too, but if Valve could have found a simple way to monetize the ads, I they would have gone that route. But this approach brings in more money, reduces cost of hosting games that don't bring Valve income, and brings good will.