[biztos]

Assuming you mean Google and Facebook et al for "ad companies" then I guess it's the very high compensation and nice work environment as well as the chance to work on super cool tech with really smart people if you're lucky or sufficiently motivated?

Of course most folks there probably work on super uncool tech with very average people, but those are also getting paid a whole lot of money and can enjoy the nice work environment, etc.

Morality, even if it were universal, is not a prime motivator in career choices for most people.