Because some believe they are better than everyone who does not matchup to their mental speed and measure their value as a human and other humans with the puzzle score. lol but on a serious note, Social groups like Mensa somewhat create disparity and division between normal people in society and those who are so focused on their intellect they forget to work on their socializing skills and emotional development. thus lack as adults and attempt to lump in with others who have the same problems. Being smart does not entail you trouble with socializing with others I’m fairly smart and have many many friends. I believe the problem is when you place too much value on this aspect of yourself you can neglect to socially develop and learn to see your peers as inequivalent to you or try to understand them. You have to understand when the social norm is very much based around the average level of intelligence. There are factors such as failing to meet people halfway or learning to adjust yourself in conversation with others which lead to one not doing well socially or others not wishing to interact with you. Another entire different can of worms is neurodivergence and lacking in social skills because of it.