Publishers want someone to handle monetisation of their website. If they get enough traffic, they want that someone to work in-house, but if they don't, they want some one (ad network) to do it for them.

Advertisers want impressions/clicks/performance.

A (successful) ad network needs to do both, so bootstrapping invariably looks something like matchmaking in the very beginning.

If you want to find my email address and reach out, I'm happy to talk to you more about bootstrapping: I've gotten more than one ad network off the ground.