It's likely that the largest advertisers don't use the self-serve ad manager. They call their rep who operates the system for them. This is why the self-serve ad managers for AdWords, Facebook Ads, etc. are all pretty unloved.
Ex-agency guy here, now client side. This is not the full picture.
Just because brands outsource media management to agencies doesn't mean the many business reasons for improving the UI go away.
As someone who has collaborated with Google engineers and PMs on such things before, I can confidently say they care quite a bit.
You see, agencies margins are in part dictated by how much time it takes to do "low impact" but "mid-high" effort activities like reporting and day to day campaign management. It takes real people hours to do those things and if to platforms are delivering otherwise comparable performance, at the agency level, they will push towards things that deliver higher margins. Which might be the platform that lets them get the most done with the least effort.
And I'd also add that i think AdWords and FB have some of the most user-friendly interfaces for such an advanced and massive feature set compared to other players in the industry.
