Yeah, people have given the answer.
He samples obscure shyt, often the artists don't have copyright or only have it in 1 or 2 countries. And underground Hip Hop doesn't sell a lot. NOBODY clears samples in the underground Hip Hop scene, because they know the sampled artists won't go after them.
However as soon as they start making serious money or they become a big name, these old artists start crawling out the woodwork and want their cut.
Alchemist is a big name in the underground, but he's still underground and the people he samples these days are either dead or completely unaware, and it would cost them more in legal fees to claim than whatever they'd get back from him.