Because it is engineered to be addictive by thousands of experts of psychology and UI design and implemented by world-class software engineers. Some of the best engineering talent and behavioral expertise is harnessed to keep us scrolling in clicking ahead whenever we have a moment available at the bus stop, on the bus, while sitting on the toilet seat, at home after work, waiting for our order in a restaurant etc. etc. It's the attention economy the purpose of which is to maximize the number of eye pairs on advertisements associated with whatever content the user wants to consume.
It has gone as far as to have Facebook developing brain-computer interfaces that would allow users to write messages by merely thinking about the words and to use augmented reality pointers. While that technology may be the future and have some immediate uses to make a world of difference in the lives of severely debilitated people, the implications of those technologies are downright scary.
There are probable few reasons that is attracting a lot of people in Social Media, such as :
Well for better or not Social Medias have become a part of our life.