CTRs do not make advertising successful. Conversions make advertising successful. Your users, when they're in Reddit mode, "consume" links at a prodigious rate and are primarily engaged in interaction with Reddit rather than interaction with the links. Conversions from them are, for most businesses, going to be too low to measure. On the other hand, conversions from AdWords are fairly predictable, because if you're doing it right you are guaranteed to be speaking to someone who wants what you're selling.
Advertising is not a magic "pennies fall from heaven" game -- at the end of the day, somebody actually has to buy something. When you cultivate an audience of rapidly anti-commercial poor adolescents, you end up with the only remaining advertisers being the sort of autoplay flash infection vectors, because scams are the only business model that works profitably.