Note the difference between "I could not find any bugs" and "I proved there are no bugs". I would assume only the latter are hired. Yet, program proving as in formal verification is a very academic specialisation. The reluctance to hire these people seems partly a) unawareness these methods exist and b) capability to evaluate whether someone knows enough. To my anecdotal experience founders of DeFi applications are not that tech savvy, so instead of trying to understand Solidity (untrivial) they instead place bug bounties or hire a special firm to give a stamp of approval for a product launch.