There are a zillion reasons why some folk move on swiftly after a breakup.
To experience the 'drug' that is the infatuation of a new relationship where everything is exciting and new again.
For lust. To swiftly replace whoever may have been previously delivering some physical / sexual need to a person.
For self justification. To satisfy some selfish, narcissistic or genuine need to feel approved of, loved or appreciated.
As a force of habit. Some people will date anyone rather than appear or be seen as being single or alone.
For romance. Some people will turn the world upside down in their search for perfection or some impossible perfect creature that will only ever actually exists in their head.
I'm guilty of that and when I eventually met my embodiment of perfection a dozen years after losing my 20 year love story of a marriage, I discovered my (new) perfect mate and match was, (as a younger 40+ grieving widow), imprisoned in a world not meant for me ... or anyone else ... until she could learn to let go of her grief.
And there are many more examples of why someone jumps from frying pan straight into the fire of a new relationship.
The only thing I would add is that you appear to be generalising when you claim men seem to find it easier to move on quicker than women. (How do you know such a thing?)
We might argue that, until the last decade or two, it was men who were expected to be the instigator or prompter of any new relationship based on a societal expectation that no longer applies in our modern 21st century world.
In any event; it hardly matters. A breakup, a real breakup, involving two people who may have invested everything in their relationship, remains a tragic and traumatic thing to get past or over.