Google and Facebook are the dominant players in the digital advertising industry. Last year, they captured around 60% of the digital advertising spendings of the United States.
Google and Facebook operate under their own ecosystems, their system of estimating the identities of the users is comparatively more accurate than other ad tech services, third parties, etc.
The increasing opacity by these digital giants is also a huge factor that protects their dominance. Google has barriers in place that hinder the entry and growth of its competitors.