The ads aren't to maximize their own revenue; that's marketing expense, not ad revenue.

The ads that drive their ad revenue are product placement within their store, like Walmart charging for a higher place on a shelf, or featuring a product in a sign at the end of an aisle, or offering it for free at a tasting booth.

Walmart doesn't make meaningfully more money when you buy product A or B (not counting their store brand), both are on the shelf, so their motivations are different.