Cause a lot of people find it a great value, and depending on their background or where they come from, the Altima's interior is quite nice.
I'll give you guys examples. In India, until recently, most cars did not have fancy interiors. They rattled worse than a rattlesnake and had generally poor ergonomics and space-efficiency (now thats all changed..but back in the 80s and early 90s it was pretty bad). Now the people who lived there come to Canada or the US and are astounded by such cars as the K-car, the Corolla, Camry, etc..
Our first car in NA was a used Dodge Aries stationwagon that went to the junkyard after a few months of owning it. It was a lemon, but we didn't knwo that (we were new here), it also put a long-standing hatred of Chyrsler in my family).
Anyway, after that we got a 91 Corolla. Pretty spartan accomodations, but it was reliable. Then we got a 99 Maxima GLE which blew us away. I still think the 99 Max GLE has a great interior (I also think its better than the one in a 98 Lexus GS300 that we may soon buy).
Anyway, if you were to put my dad in a new Altima or Mazda 6, he wouldn't think it was bad. As long as it has the features and a good design thats all that matters. Most buyers can't really find the cheap materials present. I know I couldn't until I started reading this forum and VWVortex. I still don't see what the hype is about VW interiors, it's nothing special.
I dunno about the US, but over here, a lot of the buyers are immigrants and to them, the Altima does not fall short on interior quality. Its all about relativity, most people buying Altimas/Accords/Camrys are coming from sh!tboxes. Those guys have nothing to base their interior comparision on, since their previous cars were old crappy cars. A lot of people I know who bought Altimas/Camcords/etc.. are coming from late 80s 6000LE, Chrysler LeBaron-type cars, and Ford Taurus/Escorts. To them, the ALtima is freaking beautiful.
Plus there are those that care more about the money spent in the engineering/powertrain/brakes rather than the interior materials (my family being one of those types of people).
I know its long winded, but what I'm trying to say is that you guys who say the Altima interior is crappy is basing it on your nicer cars that you have owned. To someone coming from a '86 Olds Delta88, the Altima has a sweet interior.
Btw, I read on a lot of car sites that the Mazda 6 has a cheap interior, but I think its great! Looks really nice!