You're not the only one. I'm 19 and never get mail offers other than the onrs you stated also. This is due to the FAIR ACT, it prohibits banks from soliciting people under 21 with mail offers. But from what I see it is possible to type in your info into a pre qualify tool like Cap1 and get an offer back legally. But that has yet to happen to me and I try every creditor monthly. The only one I've gotten to pop up is Creditone for a $500 card with a lousy AF. I do the creditone every so often to get an updated credit score btw haha 
But if you do have 10 cards than that is far more than enough. I have 8 totaly cards and it's really stupid IMO now because they are all 1-4k limits and barely get CLI's. I'm actually thinking about closing discover and combining my cap1 cards in the spring so that I can have less cards which translates into less hassle of keeping up with cards and higher CL's. Again the only one I got to pop up was creditone, so if you just want to see a pre qualified offer than try that.
But if you do have 10 cards than that is far more than enough. I have 8 totaly cards and it's really stupid IMO now because they are all 1-4k limits and barely get CLI's. I'm actually thinking about closing discover and combining my cap1 cards in the spring so that I can have less cards which translates into less hassle of keeping up with cards and higher CL's. Again the only one I got to pop up was creditone, so if you just want to see a pre qualified offer than try that.
Yep, it's the CARD Act protections that are keeping you from getting prequals. As a couple of other people have mentioned, that law restricts CC companies from marketing to people who are under 21. You clearly already have a solid group of cards though, so why is this a problem? Hell, it saves you from having to shred a whole bunch of annoying junk mail 
The signal to noise ratio on the prequal offers I get these days is pretty absurd; for every worthwhile targeted offer, there are a dozen pieces of garbage. Half the reason I applied for a Discover It was to stop the endless onslaught of mail though, so I guess it worked!
It seems like under-21s on here often gripe about the CARD Act, but It really does provide an important set of protections. When I was in college pre-CARD Act, CC marketers on campus were a constant nuisance, and their practices were seriously shady. They were always tabling on campus, offering pathetically tiny incentives to people who applied for cards on the spot, without making clear what was going on--think crappy pens, keychains, and highlighters in exchange for a HP and a craptacular card approval 
The cards often had fees and other terms that the salespeople wouldn't explain adequately (if at all), and a lot of people got themselves into a debt-ridden mess because of aggressive card marketing. Though I do have fond memories of taking advantage of the bank marketers who would stand outside the pizza place across from campus, aggressively pushing coupons for free pizza in exchange for credit card applications... I'm very glad my younger self was suspicious enough to only take those offers using fake names and SS numbers, so I got to eat for free a few times without any negative impact to my credit!