The main reason is because they dont have any place for the anaerobic bacteria to grow, so you dont get the gas. Ive also heard it was like too efficient or something like that, but i cant explain this one because IMO, nothing can be too efficient. Live rock has crevices deep into the rock and little pores everywhere that have very little to no oxygen. The little crevices and pores dont seem like much, but if you combine them all, it actually does something.
Protein skimmers take the waste out before it can go through the process of turning into ammonia, nitrite, and nitrate, so it eliminates all 3 before they can even form, this is why you have close to 0 nitrate. What i am thinking right now is that your nitrate and gassing is pretty close to being balanced by the live rock because a lot of the waste is being taken out immediately. If you didnt have the skimmer, you would have a lot of nitrate.
If you dont mind trying this, take out your filter floss and see if it makes any difference in your nitrate. What i am thinking is that your foam or whatever you use is catching waste that the skimmer missed, but since its a closed system, the waste will end up in the skimmer anyhow. Sure the extra media is nice, but wouldnt it be nicer to just have to clean out the skimmer daily than cleaning both?
Edit: Just to let you know, i might not be 100% sure about all this stuff, i dont even have a saltwater tank myself, but i maintain 700+ gallon systems so i guess it balances out. Im going to plop together a pico tank tonight and see how my first tank goes.