Famous celebrities and influencer's are different and we should not compare between the two. When it comes to celebrity's, they will charge you a lot for promotion of your product, ad or gambling site. Also we cannot become celebrities ourself.
On the other hand, influencer's can also help in promotion of the site however they may charge way less than the celebrities.
Also, if you to have presence on social media platforms, you can try and become an influencer yourself too. All you need is a community and community will come to you automatically if you offer a good service or education on social media.
Well, it is not always the same, some influencers manage to become more famous and popular than some celebrities and they charge a lot of money even for a 10-second Instagram story, they get a lot of fans and followers and actually have some influence around the community, most of such influencers are gamers and because gaming industry keeps growing over time, these influencers manage to get more fame as well.
And, I don't think it's as easy as you say to become an influencer, if anyone could become an influences, I'm sure everyone would try it because there is a lot of money that can be earned as an influencer if you know how to do and manage things.