Jeans used to be cotton, but these days are often synthetic. Initially, most jeans were made from 100% cotton fibres. Many still are, but with advances in technology, denim fabric is often made with a synthetic mixture. The stretch that's needed for looser fits requires material like spandex to be added to the denim fabric, for example.
There's a very moderate debate about which is better: 100% cotton denim or synthetic denim. Motorcycle pants always have some kind of protective material woven into the cotton fibre anyway, so for our purposes, it's synthetic all the way.