[virtuscience]

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.

[Nextgrid]

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.