How do the struggle so much? They have a large, engaged audience. Seems like it should be straight forward?

I think the struggle is people don't respond (convert) on those platforms for whatever reasons that are inherit to their audiences or the app itself. If users don't convert then businesses aren't going to spend on ads. I work in marketing and Twitter has always been hard to have any sort of positive return on. Big brands can afford that but the millions of smaller businesses can't.

Twitter was doing $5 bill pa of ad sales... Snap is doing over $4 bill pa. Twitter was sold for ~$40 bill, Snap is currently at $18 bill market cap.

That isn't notoriously struggling. The statement that the ad component is relatively easy is borne out by the facts. There are zero companies doing lots of ad revenue with very few users, and practically every service with hundreds of millions to billions of users spending their time looking at some service is doing significant ad revenue.

The only way one can think they struggled is if compared to the two wildly outlying, greatest ad revenue machines in the history of the world.