True, the cow was already slaughtered, and the companies already made their profit from it. but if it's used, no money is going to the company who made it. So you're not actively or really even passively supporting it.
It's also why I made sure to mention writing a letter to the manufacturer. If they see that they are losing money because of a random element in their product, add the fact that synthetic can be much cheaper to use, it could cause a bit of change eventually. I never said it was a perfect solution. Some people wouldn't even want to touch the leather in the first place.
The same thing was an issue for me when buying a used car years ago. My car broke down so I needed one asap. When looking, there was one with small amounts of leather on the interior that was much cheaper then my other choices. So I bought the car and replaced the bits of leather. No money went to the leather companies or the car company.
It's pretty much the same case with everything in modern society. Our homes caused the death of animals, our factories, our buildings, I'm sure even a rabbit or two have even been killed by a truck carrying organic veggies to the farmers market.
The idea is to not support people who are doing the harm when it can easily be avoided. And it's also a personal choice of where people draw their lines, because none of us can be perfect at it.