Attention is one of the most important things you have in your life.
Whatever you pay attention to, becomes your reality, your life.
Businesses know this very well, so they pay a lot of money to grab your attention, and therefore making you their client.
That's quite literally what advertisement means:
A notice, such as a poster or a paid announcement in the print, broadcast, or electronic media, designed to attract public attention or patronage.
People promoting gambling sites are simply doing so because they get paid by the gambling site to do so.
This happens for basically almost every product or service out there. Famous people get paid a lot of money for just saying they like X brand, even if it's not true. The masses will follow blindly and buy that because it caught their attention.
And the product itself doesn't have to be good, for example alcohol is bad for you, but it's advertised everywhere as a social fun thing to do.
It's as simple as that, they are paid advertisements. That's it.