The number of people who want to pay directly for something is far smaller than the number of people willing to view an advert. With advertising, you get more visitors, you get more revenue. And the number of visitors available is billions.
So with advertising, you can get 10x the traffic, and not need any more resources. It's truly scalable. With a 'paid for' offering, you're more likely to need to scale up customer support, handling charge backs payment issues refunds etc etc.
My comment about 'fun' was more about the overhead and pain required in managing premium payments. Cancelations, mess ups, people wanting refunds, credit card issues, etc. That's not fun (IMHO).
I've always found making money from advertising ridiculously easy compared to trying to charge people.
As for fun, money is money. I'll take it in any form.