Advertising at big companies is the only space left for traditional sales teams and it's all outsourced to agencies. Adtech vendors and publishers sell to these agencies, not the client (and if they do, its just redirected) but there aren't any long-term deals because everything is constantly shifting. Agencies have lots of teams, they win and lose accounts, work on multiple campaign initiatives and strategies with constantly changing budgets and requirements, and use dozens of vendors to create and execute campaigns.
This is the opposite of a traditional SaaS contract sold for a term directly to the people that will be using it. There's also a steady trend towards all inventory being traded programmatically which will eliminate most of the sales negotiations. Unique inventory and formats can still stand out but they also cost more in effort and money so there's less overall demand. It's very hard to scale custom sales like that, especially if one of your requirements is to avoid all the javascript verification, brand-safety and measurement.
Most media sales these days is more about biz dev to get the pipes connected to exchanges and represent your inventory in the best way possible while letting the market work.
