Part of the reason is because for the longest time, the smallest unit of "value" recognized by most people was 1 US Cent.
Crypto currencies, like Dogecoin are intrinsically more divisible.
If someone made a funny comment on the internet, and i wanted to reward them.
In 2012, I might "like" or "upvote" their comment -- a subjective measure of appreciation.
In early 2013, I might "give gold" or "gild" comments or posts I like on reddit.
In 2014, I'll probably just give some Doge, or whatever the new popular currency is (which I can easily buy with my other altcoin/currencies online). I can also give more or less based on how much I want.
This wouldn't work with pennies or USD because 100 Doge still __feels__ like a lot -- even though it's some fraction of a penny.
For a long time, people would give each other "internets" to reward what they like. The only difference now is that you can buy internets with real money, and they retain at least some quantitative value.