I think DRM or any other copy protection method are implemented not to sell more copies. But to build trust to the developers and in case of the marketplace ... to give trust to it. I totally agreed that every system can be easily cracked, but I think that many developers prefer to have a kind of copy protection system rather than not.
Although Itch doesn't provide any and developers sell their games here as well. So many devs don't care so much about copy protection, I guess. Some DRM systems, in fact, can be invasive for gamers