Why do you never see any other planes in the sky whilst flying?
If you're not seeing them, you're not looking. 

I see planes all the time when flying commercial.  Obviously, I see them a lot more when close to a major center, and a lot less when in the sparse parts of the Pacific Northwest (Montana, Idaho, Oregon, Washington).

I think spotting them may take some practice. Visually, a plane 10+ miles away is a very small dot, and easily missed.   Try looking for contrails and be sure to look at altitudes above and below you.  Scan the sky in narrow, 10&deg;  concentrated "fields of view" to scan from left-to-right all the way across the sky.