Its now like a natural phenomenon because its happening everyday. I think the reason could be some of those listed below
CONNECTION AND EXPLORATION
People want to connect and as well explore. Connection is what majority are seeking after in other to get what/where they want or increase their business or whatever. At the other hand, exploration is more or less using your connection to venture wide. In other to achieve these things, what you know can't really be enough. So you must be known or strive your best to know people.
Trust is another thing that brings us close to people and also makes us known. You might not be academically sound in an area you'd like to venture. But based on the trust someone has on you, you can be taken. Nobody wants to have an employee that's not reliable. And nobody wants to take the risk of trusting a stranger at the detriment of his/her assets, so the other person who's known, tested and trusted is taken.
EXPERIENCE/QUALITY OF SERVICE
This is basically what people are looking for in terms of service delivery. Everyone wants someone who can deliver a job as stipulated without compromise. And as such they rely more on those who they've been using than those who claims to know about the job. But at the other hand, the person who claims to know per say might give a good job or a better one than the first person.
People wants to favour their relatives/friends. They do so by giving them jobs or opportunities meant for those who really deserve it. And in turn progress is usually little or nothing at all. Favouritism is one thing that has hindered a lot of development in Africa, more especially Western Africa (Nigeria). Employment is more on who you know than the very person who's good for the job. E.g a lawyer in charge of ministry of power,etc. What knowledge can the person deliver apart from hiring experts.
There are other reasons apart from the aforementioned. Its pretty bad though, but there's no choice. I believe that's why, the world is what it is today.