Of course you can assemble a better kit.....but would you do it for free?  After factoring in the costs of say Leatherman vs Winchester AND a profit, who would be able to buy it?
I would dare say that 99% of the commercial kits available don't contain "THE BEST" of every single item in a kit.  For the sake of affordability,  compromises have to be made.  While I wouldn't personally buy a Winchester multi-tool, I would imagine that it would suffice for most cases for the 24 or 48 hours until rescue.  
If it is that easy, go start a company, make kits and get rich.  Anybody see Doug Ritter's new Jag?? <img src="/images/graemlins/grin.gif" alt="" /> <img src="/images/graemlins/grin.gif" alt="" />