To be fair, in the case of Uber, at least in London there's always someone paid to stand near the metro stations distributing Uber Eats vouchers.

Of course, Uber's objective is to acquire you as a customer and not give you free food, so it's limited to new accounts only and presumably they check credit card and phone numbers to prevent multiple accounts being created.

However, if you try hard enough, you can indeed take advantage of it and automate the process assuming you have a good source of phone numbers and non-fraudulent credit card numbers.