They don’t clear 90% of it man. Sometimes they have a personal relationship with people they sample and get permission, but they don’t do the paperwork sample clearance game.
@mr x is right in that you have to, legally. But if you’re fine to skirt the law it’s rare that you run into trouble when you’re just selling a few hundred pieces of vinyl, tops. Most people if they find out they’ve been sampled on these type of projects don’t even bother pursuing it. The costs of litigating it are less than what they’d even get on an album like Tea in China.