As I wrote elsewhere, I believe there are still cases where leather really makes sense from a comfort/durability standpoint.

I, like most people, still eat at least some meat, so it seems rude to not make good use of the rest.

I mean, that is extremely easy to answer.

1. It was an otherwise-wasted material that is already bonded, minimal tools needed for processing,

2. In colder climates, fur was already attached to the leather which means that warm clothes could be crafted easily, fast, and by gathering all of the materials at one place. In early historic times, growing suitable plants and having the capacity to process them, strip them, straighten them, and weave them was not always possible and would be a full time job where it was, which means less time working in agricultural fields. Plus, animal fur has bigger growing seasons and is much warmer than any plant alternative.

The real answer for the modern world is that leather is a byproduct of the very shitty industrial meat industry. The meat industry already has more leather than they can get rid of and it regularly goes into landfills. From an environmental standpoint, it is much better to use the leather than almost all other materials used for those tasks which burn oil and use oil to create them (every leather alternative is simply shitty petroleum-based non-recycled plastic, every one). From a moral perspective it is shitty to give more money to the meat industry, but they make the vast vast majority of their profits on the meat and not the hides. If everyone switched off of leather right now, the same amount of animals would be brutally murdered, but instead their hides would be dumped in landfills and the ocean after stripping it off of the meat.

This isn't even getting into waterproofing. Almost all clothing waterproofing is petroleum or animal oil based (wax is either petroleum or harvested from bees) except for natural rubber which also has a bunch of negatives.

Because they won't let us use human skin for seats in our cars like we all actually want. You know regulators, bunch of bitchy little girls.