Snapchat requires semi persistence for stories (viewable for 24hr) and millions of users can view a single story. When a celebrity posts a story and 5mil users view it that requires more engineering effort that p2p what’s app messages. Additionally, serving ads requires a large amount of compute (ad ranking/serving), storage (impressions), offline processing (Hadoop clusters are hard), engineering, salepeople, economists, and auxiliary systems to support advertisers buying ads, etc.
As an example, check out snaps blog post on ML and ads https://eng.snap.com/en-US/machine-learning-snap-ad-ranking
Don't get me wrong I think you're probably right but snap's business model is not just a pure tech issue at scale, i.e. you need old fashioned sales