Why are guys so stupid?

Does someone understand why guys never mature? I have an ex that is about to turn 20 and hes always left me for other girls. I’ve made the mistake of always taking him back after that thinking that he would change, that he loved me and because I love him. I don’t understand why guys would throw something long term and special, 5 year relationship where we did everything together, saw each other grow and were there for each other. I don’t mean to be cocky but I considered myself to have been a good girlfriend. I was always there for him, supporting him, loving him, caring for him, making sure he’d eat, make him food, always cheer him up when he was down. I was always there. I did my best to make him happy but he’d always leave me for other girls. I don’t know how he can just leave me, throw away all of our memories for someone he met at a stupid party. It’s like he never valued everything that I did for a him. Idk why guys would want to throw something good and real for a person they met. It sucks because now a days there are many girls that just use guys for money, if they got a car, or just to play around. But I’ve been so loyal and my love for him was pure and real. I took him back despite him cheating, disrespecting me, leaving me for others and it’s like he never learned that I deserved to be valued and that I’m rare☹️ it’s been two weeks today, he cheated on me and is with her now. Sorry for ranting so much, don’t really have many people to talk about this 😐