When you sleep on your side, one breast falls onto the other and the skin in the middle of the chest gets compressed in the same direction, in the same fold, for 6 to 8 hours straight.
You repeat this over 300 times a year. The skin learns to fold.
First it fades in the morning. Then it stays. That's when the vertical lines become permanent.
That's why creams, serums, retinol and sunscreen do little here: they treat the skin's surface, but the cause isn't chemical, it's mechanical.
The nighttime compression that creates the fold. Without stopping the compression, nothing else helps, not even cosmetic procedures.