Advertising makes up only 9% of Duolingo's revenue and is also usually a small part of the revenue of most mobile games. Mobile games rely on "whales" (people who pay a lot, like a casino) and Duolingo makes almost all of its revenue on subscriptions. Both require sticky retention but advertisers don't drive the business model or executive decisions at these companies.
Absolute percentage of revenue (or for that matter, profit) is irrelevant. If employees are able to justify their salary/promotion by increasing a metric such as ad impressions or "engagement", you're gonna see more advertising, even if it affects long-term profitability or even kills the product.
