Older materials can be out of print, and thus impossible to get any other way.
In print stuff? Makes sense to not pirate (even wotc needs cash flow to function). But stuff that isn’t, it either gets archived and spread via p2p/piracy, or it dies.
Just as an example, I have the original box sets. They’re still usable even, because I’ve taken pains to make sure of that. But if I wanted to run a game with that stuff, it wouldn’t take much for it all to fall apart. It’s irreplaceable without digitizing. And, sure my kid is going to have the choice to sell or keep it, but other people’s kids won’t. Losing the material is a much greater loss to the world than whatever downstream affects piracy of that material may bring.