They do. For good reason. Cena sells PPVs and keeps ratings up. The WWE and WHC Titles no longer do. A combination of the big stars being gone and the newer stars unable to get over to that level, and the WWE hot-shotting the world titles around so many times for so many years that most fans really don't think those titles are nearly as prestigious and important as they used to be. They know the Champ won't keep the title very long so they aren't as willing as they used to be to invest emotionally in the Champ.
We can't "blame" this on Cena. He's just doing his job and the fans respond (positively and negatively) to everything he does because they care about what he does. They care less about who the Champs are. We can blame Vince's crayon-wielders for that. Vince, too. And Triple H.
Prime example and we see it unfolding every week: WWE Champ is Punk. His main rival is former WHC Champ Daniel Bryan. Unquestionably the WWE's two best in-ring performers who have had some great matches recently. Both guys are over; the crowds respond very well to them, and the internet talks about them constantly. Both guys ARE over...just not "Cena" over. The WWE hasn't given them time to get over to anywhere near Cena's level. But we have a classic rivalry, the two best wrestlers on the roster battling over the World Title.
Vince and the crayon-wielders toss Kane and AJ into this feud. Why? Punk and Bryan don't need them, they can carry this feud themselves. We CAN see the "logic" in adding Kane; he's a former World Champ and he does have a beef with both guys. But why AJ? What does she add to the feud? Nothing.
What AJ has done (not AJ herself, but the crayon-wielders by how they're "writing" this stuff) is devalue the WWE Title, "the richest prize in the game" as JR says, and elevated AJ as the real prize in this feud. She makes an appearance and all three stumble over themselves to get noticed by her, or they stand stock-still mesmerized by her, and the WWE Title is forgotten. The commentators are pushing AJ as the prize over the WWE Title, as well. Basically, we have three guys on the schoolyard playground fighting over a pretty girl who's loving the attention and doing everything she can to keep them fighting over her. The WWE Title is an afterthought.
Meanwhile, the grown-ups are taking care of the real business. Cena (because this discussion is about him) is defending his reputation, credibility, and status against a monster (apparently Triple H has taken over that fight now), and fighting to get rid of a power-abusing tyrant, after facing one of the biggest stars in history.
Cena is taking care of adult business and three schoolkids are fighting over a girl. Cena doesn't need a championship, and the other three value the attention of a "crazy chick" more than the "richest prize in the game".
Yeah, the WWE is still relying on John Cena to carry the WWE, to keep selling PPVs, and to keep ratings up. They don't seem to have faith in any other "superstar" to do what Cena has done the past several years. They're not giving Punk and Bryan the chance to establish their own "dynasties" ON their own; they keep over-booking ridiculous stipulations, angles, and storylines involving those two. The WWE Title has become just an afterthought.