Because MOND doesn’t really explain the data. For one, it doesn’t really explain all the mass but for another, the bullet cluster is kinda a death sentence for MOND. Like good scientists, we can’t rule it out completely yet but dark matter explains all the discrepancies much better than MOND and so MOND is a fringe theory among physicists. In addition to the Bullet Cluster, MOND also poorly fits several other features which dark matter explains naturally, including much of cosmology. Less damningly, MOND requires a rather complex, random looking change to physics while dark matter is actually quite simple. We are pretty convinced there it is possible there are particles we haven’t seen yet so it’s not terribly surprising some of these could have astronomical effects. Also, I suspect you have a hard time grasping much of physics (QFT and GR for example) but QFT has been confirmed at the highest precision of any scientific theory ever. The fact laymen struggle with some physics isn’t a good reason to believe something else.