It's not legally mandated, but the dynamics of the regulation and the risk-averse nature of companies mean that it's effectively become a requirement to compete: if you don't have it, you're only going to sell to the rare company that is willing to stick their neck out and deal with novel arguments in the paperwork themselves. For commercial aerospace that is none of the manufacturers.

(someone else might come along and certify it themselves, effectively acting as a middleman, but then they're going to get most of the money)