As others have said, it’s to absolve themselves from blame. If they find enough faults with you, it excuses them.....
When I got the “I don’t feel the same anymore” speech, the list of my crimes was ridiculous. The things thrown at me included:
Brushing him off at 2am one night, three months earlier , when I was tired, so “rejected him”
Not wanting sex in a PI when had especially bought a condom from the machine (he fell asleep)
Not having sex for almost a month (he had cracked ribs and pleurisy)
Wanting to go on holiday, he didn’t allegedly (yet jetted off with OW a few months later)
I didn’t have tea on the table when he came home
The house was untidy
...... the list is endless, but it was all petty things to excuse why he had to leave. Once I became the evil monster then poor little him couldn’t possibly be expected to stay with me. (Of course the fact he was texting OW had nothing to do with it, they were just friends. )
Reader, he married her 😂😂