Comment by brigandish

Partly because devs tend to labour under the misapprehension that:

a) pretty much any closed source project or online service can be done just as well by gluing together parts of open source projects…

Which may be true in some cases, but the real misapprehension is the final part:

b) …and it won't be difficult to do.

The classic is the HN comment about Dropbox, but I see at least one comment a week saying something similar. No doubt there are many more.