probably some of them are just too lazy to explore on the web to find some other ways that has a potential to pay them a decent amount or some of them just don't have enough time for it . but that is definitely true, as long as we have an opportunity to earn more from different ways then we should grab it and do not stick to only one way if we really want to achieve our goal .
Exactly, being dependent on just one method of earning bitcoins wont really make you reach the target you have set for yourself, someone would probably have to find other ways of earning bitcoins as well along with their one method so that they can achieve their goal earlier.