Simple answer… because it’s too exhausting trying to maintain a facade.
Whether you’re trying to fit in, impress people, or want what others have, being yourself is always the best route. I’m not saying people can’t improve themselves. If you’re an asshole and aware of your assholery, yeah, you can do something about it if it’s impacting your life in a negative way. Let’s admit it, some assholes get away with things and their life seems to go smoothly. At least they are being themselves… in a punchable way.
Being yourself is something you don’t have to keep track of. If you keep switching personalities based on who you’re around, you have to keep track of that mask and who you wore it for. Keeping up a front is not easy. You’re bound to slip up and reveal yourself. When you do, will those same people like you? Will they be shocked? This only adds more confusion, self-doubt, and possibly guilt.
Let people know who you are upfront. If they like you, they like you. If they don’t, is it really a loss? It’s OK to not impress everyone. It’s OK to not become friends with everyone.