Why do men move on to new relationships so fast??

Hey all, I need some input. It might not matter because I was the one who ended my 3+ year long relationship end of May this year, and I am over it for the most part and knew it was over for a while. But how come men will drag their feet, not even doing the bare minimum at the end of a relationship and then all of a sudden months later it seems they are the perfect partner to someone else? Ex and I 2 years of a perfect relationship where he treated me so well and gave me the world- we would go out on vacations, hang with friends, have fun, laugh, etc. it got to the point where right before covid we had a Disneyland trip planned where when it was cancelled he had said it was where he was going to propose. I had also gotten off my birth control and we were going to try for a family. Well maybe it was a blessing that none of that happened because when covid hit and all those plans were dashed and we were forced into our home, things got bad. We weren’t communicating anymore the sex went out the window. We disagreed on a lot of stuff regarding covid and started seeing differences in world views as well. We broke up in January of this year then decided to try one more time, but the months between January and end of May was basically me trying SO hard to make it work, trying to get him into therapy (I had been going for 6+ months on my own) which he refused. I couldn’t even get that man to go on a 10 min walk or a date night with me…. So finally I told him like are we done or what? because I don’t feel like I am a partner in this relationship and don’t feel any effort on your part.. and he basically gave no objection and so I moved out. Well fast forward 2 months later and while I am trying to find myself, focus on me and what I want, making plans to move out of the city with my best friend and experience the single life that I never really got during my 20s (I’ve been in a relationship pretty much the whole time from 19-26). He not only has a new girlfriend, but is FLAUNTING it on social media. FB official, long sappy love posts about how they are “finally with their high school sweetheart after so long” “sometimes your soulmate was right in front of you and you didn’t even know it” and most recently matching tattoos after a month of dating. I know I shouldn’t care and focus on myself but it really hurts. Especially when your relationship ended because of one person not trying, refusing to meet your needs or make the tiniest bit of effort, and then his new girlfriend of a month is posting about how he treats her like a queen and she “never knew a man like this existed.” Well I had that man and he disappeared for the last 6 months to a year of our relationship and I’m now left wondering what she has that I didn’t, to make him become that person again. I know I wasn’t perfect and struggled with a lot of depression and anxiety during 2020 that I could sense he was sick of dealing with, and from what I’ve seen this girl is kind of the opposite of that… but Idk. I guess I’m starting to feel less hopeful that I’ll find someone and this isn’t the first time this has happened my ex of 5 years moved on to a new serious relationship 3 months after we broke up…. Going to start calling myself Good Luck Chuck at this point.