It’s a stereotype that designers love to design but hate everything else that comes with it – collaboration with clients, dealing with sales, marketing, etc. Though there’s a grain of truth here, it’s mostly an illusion and result of bad processes.
To make good banners, designers need directions. Designer’s job is not to make things pretty but to fulfill some business goal (increase revenue, raise awareness, make a conversion). But the thing is, the people who know the most about fulfilling those goals aren’t designers, but marketing. This is why designers need an effective design process that supports collaboration between them and marketing.