Their CAC (Customer Acquisition Cost) is usually very high, just think about it. Aside from the subway ads that have ridiculous costs, they usually offer youtubers the equivalent of a year's subscription for each conversion. They also have to factor in operational costs so they've to keep new customers around for 1,5-2 years to reach break-even, only after that they've profit. Their business is based on a CLV (Customer lifetime value) that greatly exceeds the average for a B2C digital service - in order to survive they're either doing shady stuff with your data OR living of VC hype money.