Old, yeah. Slow, hardly. Sucks? WTF are you watching? The dude only had one loss (a DQ loss at that) in an an eight-year span. He had a 17-fight win streak that included ten UFC MW title defenses. He had an incredible finish rate, and beat BJJ black belts, Olympic wrestlers (guys who were on the Olympic teams), and MMA champs in the UFC and other organizations. He's got more wins than everyone else in the UFC MW division, he was the first guy to beat Sakurai, and held titles in Shooto and Cage Rage. I don't understand how anyone can look at that, can see arguably the best striker in the history of MMA, and say that he "sucks".
Nor is he "totally done with MMA". His two losses in the UFC were to the same guy, which might just mean than Weidman, who has yet to taste defeat, simply has his number.
So how is he ranked higher? It depends on which ranking you're looking at. Fight Matrix has Hendrix at 8, Pettis at 9, and Anderson at 13:
http://www.fightmatrix.com/mma-ranks/pound-for-pound/
Bloody Elbow has Anderson and Pettis ties at #7:
http://www.bloodyelbow.com/2014/5/7/5691504/sb-nation-mma-rankings-pound-for-pound
Sherdog is similar, having Pettis, Silva, and Hendrix at 7, 8, and 9 respectively:
http://www.sherdog.com/news/rankings/Sherdogcoms-PoundforPound-Top-10-65261
Of course, MMA Junkie and the UFC have Silva ranked above them:
http://mmajunkie.com/rankings
http://www.ufc.com/rankings
So what does it all mean?
Nothing. Absolutely nothing. The "pound for pound" ranking don't affect title shots, who earns more money, who has titles, etc. They're entirely the product of imagination: who is the best fighter, overall, if everyone were the same size?
I can only speculate why someone would consider Anderson a better fighter than Pettis and Hendrix. Hendrix is very limited in his skills (great TD's and a hard left hand, but that's it) compared to Silva, and doesn't have nearly as many top-10 wins. Similarly, Pettis hasn't had the number of top-10 wins that Silva has had. Neither one of them appear overwhelmingly dominant compared to Silva, and it's very possible that they'll both lose their titles before they make it to 10 defenses. As well, during his UFC run, Silva moved UP in weight three times, even defeating a former LHW champ. Rousey, compared to Silva, may not have the quality of opposition that Silva has had; her last opponent was practically a rookie, as was her defense against Liz Carmouche. She hasn't yet had even ten fights, much less ten title defenses.
I could make arguments against Silva as well, just like you have. But you're asking why someone might rank him above those you listed. So there you go.