I cant explain the scientific reasons behind the burn in on cables.
I have listened a lot to new cables breaking in.
What usually happen is that first it sounds compressed with lack of deep bass, its also sounds more harsh in the top.
After playing them the bass will extend, the top will get more smooth, and the last things that happen is that the sound open up with a holographic soundstage.
Sometimes it can sound pretty good from the start and sometimes it sounds awful. Some cable gets better and better, and some goes through changes where they can sound good, and then worse to the point i thought something is wrong with the cable. Some cables takes extremely long time to break in, specially high-end interconnects and digital cables.
I have experience that break in took 3 days and some 2 months.
The changes are more profound then simply subjective opinions. Its very much real. If you are willing to go down the rabbit hole, and test as much as I have done you will discover this. I usually dont discuss cables on forums online due to abuse, but if you are interested in this topic I thought this could be of interest.