It's one thing to build a WhatsApp-type app, focusing 100% of your energy on the core messaging functionality itself, and little else.
It's a completely different thing to build a revenue-generating business that needs to connect with an entire ecosystem of advertisers, ad servers, video servers, programmatic ad exchanges, analytics, brand safety, reporting, billing systems, and a lot more.
And we're not even talking about services. There's another big layer of complexity to keep those services running 24x7 (well beyond just SREs), ensuring ads are running properly, campaigns are fully optimized, advertisers are happy with what they're getting, etc.
Of course ideally the question would be “why not just have users pay you directly instead of relentlessly monetizing every bit of personal information you can squeeze out of them” but as an industry unfortunately it doesn’t seem like we’re there yet.
My interpretation of the GP's comment is that the advertising platform is not the core business but rather a secondary concern. Focus on your core business and outsource the rest.