The common theme that runs through here is that, regardless of your age or level of technical ability, most people admit that their skills were passed along from someone; either a parent, sibling or friend.
People gravitate toward forums, such as Cycle Chat, because they know that they will find like-minded and, one assumes, knowledgable people, who are willing to give advice. In a way, that's no different to asking your dad / brother / mate to show you how to fix something - the only difference is that we now have a channel that lets people do that on a much wider scale. It's social learning and, if nothing else, is a great way of making sure that these skills are shared and preserved.
Equally, let's not forget, that for the majority, the role of the bicycle has changed. For many (but, I accept, not for all) the bicycle is a more recreational item than it was for previous generations; go back a few decades and for many, it was primary trasnport. As I say, there are a great many who do not fall into this category - and as someone who recently put the car up for sale because I cycle or walk everywhere - I count myself in this number. But, if you need to be able to fix your bike to get to work, you have a lot more incentive to be able to fix it yourself than someone, who uses it for recreation / fitness.
Having said this, I do honestly beleive that everyone who slings a leg over their bike should have the basic mechanical skills to keep it rolling - especially when they find themselves 40 miles from home and wondering how to fix a basic mechanical failure. But, if those who don't know want to learn then I certainly don't mind helping - even if that help is offered through a keyboard, rather than over a cup of tea in their shed.