Historically, because they don’t want to piss off their brick and mortar retail partners. Someone like Walmart would just stop carrying their product if they’re getting a raw deal out of it. These days, Nintendo is doing exactly that, but at some point, if Sony’s charging $90 physical for a $70 digital game, you’re already convincing your customers to just stop buying physical anyway, so they’ll also save the money they’d have to spend on manufacturing the disc drive in the first place.