Why did my ex move on so quickly?

Different tack: I don’t think you asked what you want to know, you asked what you’ve heard other people ask.

Such as, if your ex moved on fast because they never actually loved you;

and so on.

When a dog smells another dog’s poo or wee, do they then remember that scent for when they smell it again, or even further know which dog they are smelling if they know the dog?

Of the ones I was able to talk to about it all rationally later, some of them admitted they “used” a new “hook up” the same way some people use drugs, to dull the upset over our breakup; some said (perhaps as revenge) that I was easy to replace; some said they just got lucky. Most didn’t stay with the quick replacement, but a few even “panic married” the replacement, as part of being upset about the breakup.

I know from my own “misadventures” in love, that my ex’s were able to move on quickly because they were more attractive than I am, and far less shy, and sometimes because they were far less picky than I am.

or if they moved on fast because they can control their emotions like a simple light bulb;

If you’re an atheist, what would be your motive in spreading atheism, and why would you care what others believe?

or if they moved on quickly because they were secretly starting up with their current person before you broke up;

What I suspect you really want to know, is one or more specific questions.