Gambling companies must spend money on promotions to attract people to them and get more gamblers. this is their business and everyone wants to grow their business by various advertisements. Similarly, advertising on TV channels is one of their marketing strategies. and it is very effective because when they promote their casino site banner among different sports, all the sports fans are encouraged to play for their favorite team. and gradually they started engaging in gambling
People used to watch TV channels more often when the internet wasn't as common and accessible as it is today. Nowadays, the percentage of people who watch TV channels has reduced greatly because most people use the internet and watch movies and shows and other stuff using streaming services which are usually ad-free which means that there are no advertisements shown to subscribers in most of these services.
That's why, I believe that most gambling platforms and casinos use online channels for marketing purposes and to promote their products and services. Social media platforms are very much popular for such things, other than that, websites that put Google Ads in their banner positions often get gambling ads to promote.