There is a Valve policy against integrating ads on games

A Valve policy that has existed since the year 2020 has been given its own proper page. This “new” policy protects Steam users from having to deal with egregious advertisements in Video Games they buy. To protect the store from becoming a mess like the mobile gaming landscape, this was a needed and obvious thing to do. Despite some bad things Valve has done, this one is a massive win. We at Infinite Start will cover the details

No ads served

According to the Steam Policy itself:
“Steam does not contain any paid advertising, nor are advertising models supported in games distributed on Steam. There are varying interpretations for what constitutes ‘advertising’ in a game, so the examples below are meant to help guide developers as to what is and is not supported on Steam.”

It is actually possible to advertise something as long as the following are taken into consideration:

Product Placement

Games may contain real brands, products, personalities, etc as part of gameplay, provided such portrayals are not disruptive and are appropriate within the context of the game. For example, a racing game might feature real life sponsor logos on its race cars, or a skateboard game might include characters wearing real-world brands. Note that all developers must obtain the relevant permission and/or licenses for any copyrighted content contained in their games.

Cross Promotions

Developers can run many different kinds of cross-promotional activities on Steam, such as bundles, sale events, and more. Promotions may involve other Steam products, or products/brands outside of Steam. These types of promotions are encouraged and many customers find value in them. However, under no circumstances is it okay to charge other developers to participate in a bundle or to sell access to a sale page or other page on Steam.

Paid Advertising Outside Steam

Developers may want to run paid advertising campaigns to drive traffic to their Steam product pages. We recommend taking advantage of Store Widgets and UTM Analytics to help enhance and measure the effectiveness of any off-Steam campaigns.

However, the usual way to serve ads is not allowed:

Developers should not utilize paid advertising as a business model in their game, such as requiring players to watch or otherwise engage with advertising in order to play, or gating gameplay behind advertising. If your game’s business model relies on advertising on other platforms, you will need to remove those elements before shipping on Steam. Some options you could consider include switching to a single purchase “paid app,” or making your game free to play with optional upgrades sold via Micro transactions or Downloadable Content (DLC).

Developers should not use advertising as a way to provide value to players, such as giving players a reward for watching or engaging with advertising in their game.

Developers should not charge other developers for access to Steam features. These include sale pages, bundles, store pages, franchise pages, etc.