Because access to that resource can be replicated by simply viewing network traffic logs. Sending authorization info in the headers over SSL would be much more safe.
URL based Bitcoin wallets proved this not to be the case. URLs get picked up by Omnibar, Skype, etc. they find their way into search results... I wouldn't even trust secret material in the fragment-id even though that in theory is safer.
The dodgy ones don't have access to the URLs. The example was Skype links: if Microsoft scrapes those, they'll follow the rules and not make them available in searches.
If you give your link to a dodgy search engine, you've lost.
The path is exactly as secure as authorization headers. Network logs will not show the path of SSL requests (it's encrypted).