A few reasons - but high level Google and FB didn't always support easy remarketing integration, hence a lot of big media agencies and companies are "grandfathered" into using services like AdRoll. It takes businesses a long time to switch service providers (and media agencies especially for a number of industry structure related reasons).
The same thing happened in Search Marketing software - initially lots of 3rd party companies moved in to offer software which enabled services beyond what Google enabled directly (Marin, Kenshoo, Efficient Frontier, etc...). Overtime Google has significantly increased the arsenal of tools available when buying direct from them, making it really tough to be a 3rd party search software provider, as you're basically just increasing the cost of the media.