Maybe because you keep on holding on to something that will never happen again

It's a number of things.

One part can't believe it's really over/they are going to come back and say just kidding...

one part afraid to move on, for fear that that would prove it wasn't real, and the Love God will get angry and make sure you don't get any more...

one part hoping the love god will step in and say "Now wait a minute, that wasn't what I told everyone to do!"

one part resentment: "I'm going to stay right here, until love admits that it LIED to me."

And of course, there's just straight up mourning. Nothing dishonorable or shameful about that.

And sometimes there's embarrassment, that we let love go, or that the other person fooled us into thinking it was there, when it obviously wasn't.

So many little bits and pieces.

common denominator - One Part

Because in your life your brain forms a picture of reality.
Your brain does not like to expend energy.
Change forces it to do work.
No one likes to work unless it's pleasurable.
Forced change is not pleasurable.

Rewiring your brain is a lot of work.
If I say the word "gray" do you immediately think of "elephant," and then kind of smile at cute baby elephant pictures in your head, like Dumbo?

When you break up with someone, or someone breaks up with you, you are ultimately now telling your brain "don't think of elephants when you hear 'gray,' and if you do then you have to feel bad because elephants are now a source of pain."

When you break up you don't immediately have a relationship you can slip in there and reattach those associations to.

So over time you either build up a tolerance to them, substitute associations that distract from them, and/or you build new associations as a new or better source for what pleasure they gave you before.

Like some people are in a relationship for 20 years without getting married, or were married for 30 years, then they get a divorce but are married again to someone "uglier" or "meaner" or "worse" a month later, or they just go back to old relationships, or spend more time with friends and family, or become helicopter parents. Substitute.

Or some people go off on a bender and get drunk to "numb" themselves, distract and build a tolerance.

Some people go off and find hobbies, or go back to college to finish out a degree and focus on a career.
New associations.

In any case it's hard to move on because it's little different than having your dominant arm broken.
You can't sit there and stare at the walls waiting for it to heal.
The pain is there to push you to do something.
Go to a doctor, get it set, put in a cast, take some pain meds, learn to do things with your other hand, have patience, get the cast removed, do some physical therapy to get it back to normal use.

It takes time and effort.
Much easier to say "well, if I don't move it this way it doesn't really hurt that bad, so why can't I just do that, why is it so hard to just get it fixed and why should I?"

Ultimately what you get is your brain is all "aaagh! I hate work! Stop making me do stuff!...C'mon, don't you just sit there, go do stuff so this stuff stops hurting! I'm going to keep thinking about this painful stuff until you do stuff that makes me stop thinking about this painful stuff; but I hate it when you do stuff that makes me do more work so do something pleasurable, and the best pleasure was the relationship that just ended."
That causes a lot of stress and pain that feels like it lasts a lot longer than it does and that it's more important than it is.

Or IOW your (general your, not you specifically) brain hates you (general you, not just you specifically) and is a prodigally intelligent childish hypocrite.

Because it is easier to do nothing
than to do something about it.