My personal theory: same reason but scoped to a limited domain. You can be a generally happy and successful person but a fanboy of an athlete because you've never achieved much in sports. You want someone to succeed because it resonates with some unrealized potential in yourself.
Judging from the people who have self-identified to me as fanboys: They often aspire to follow an example. So the difference between the groups may be less how much they have achieved and how much they hope to achieve -- a question of optimism as much as anything else.
Being either seems like a lose-lose situation to me. If you are not being critical in your evaluation of someone you may gain from, then that's wasting time. I have found a reasonable way to combat that by always considering how much I would have been paid on average if I tried to work the time I spent. Currently the amount is hypothetical but I think that may change when I get stable income in future. I notice that aspect of thinking matures some people.
So if people become haters because they never achieved much, why do people become fanboys?