People buy followers because it is an easy and cheap way to look credible and people tend to follow those who are already popular. Here is an example for Twitter:

Anybody can afford this little money and it can give them temporary boost. It's just prole thinking short term and taking the easy path. These people need to learn about the Marshmallow Experiment: https://steemit.com/psychology/@vimukthi/the-stanford-marshmallow-experiment-and-its-implications-on-the-economy-and-democracy