It seems prevalent within organisations where people have little contact outside their immediate business connections. People who choose not to network are somehow cutting themselves off from the benchmark against others.
I don't think all networking organisations and opportunities are great. Some of them are definitely not for me, but I enjoy the ones I do get involved in. The people whose paths I cross work in many different fields and - no matter how hard one might try not to be judgmental - they range from dynamic to a bit dopey.
The dynamic ones sometimes demonstrate behaviours not to be repeated and the dopey ones can show how humility works. There really is something to learn from everyone. Which is my point, really. If you're not getting out and about, you'll believe everything you tell yourself about your business and, indeed, your professionalism. Whether that's good or bad.
I think it's almost impossible to be professional in a silo. If you don't converse intelligently with people other than your clients or your colleagues, you won't ever learn anything new. And if, by the way, you think you have nothing to learn from anyone, you really aren't professional.