My parents had APs a couple years ago and we were quite shocked that they were paper. WDW is the only theme park I've ever seen that didn't have plastic cards with pictures on them. I voted that I was afraid that they'd wear out, simply because I've seen it happen. Not necessarily on an AP, but on a park hopper. In 2001, I went to WDW and it rained every single day of our 7 and a half day trip. Our tickets (on the same paper/cardboard/whatever) tickets as the APs looked like hell by the end and no longer scanned. Regardless of what they're made of, if it's not plastic, it's not gonna last forever. The sleeves help, but in a torrential downpour that even soaks through your water resistant backpack, the ticket will get wet.
I have worked at Six Flags (yeah I know, they're not really comparable to Disney, they're lower quality, and they're considered the enemy) in season passes for 6 years. For the past couple years, I have been one of those in charge of season passes. The plastic cards we use cost about a dime a piece. The printers are expensive to an individual, but affordable to businesses (we have the tightest budget I've ever seen, if we can afford them, I'm sure WDW can). The resorts use printers like our old ones, but we purchased newer ones in 2002 that are smaller and quicker. I don't see why they can't have one printer at say Guest relations in the parks to print off APs. It wouldn't really take any more time to do (maybe the time for the cm to travel to a different computer hooked up to that printer. I don't know about WDWs system, but with ours, we can only route each computer to one printer and I'd assume that the primary printer would still be the "regular" ticket printer). The pictures are usually crap, plus WDW has the biometric finger readers, so pictures wouldn't even be necessary. There wouldn't be any more labor involved (unless demand is high for APs and there were to be one window for AP's only) and all that would be necessary is a new printer per location (and by location I mean park guest relations). Signs directing future AP holders would also be necessary. Time wouldn't even need to be a factor. The process to even take the picture is very quick. We can process a pass (picture, print, all of it) in less than 30 seconds.
I'm not trying to complain. I don't even have an AP, so I really have no right to complain. But, that would be why I'd want something seeming more permanent. The above, is not meant to go against any of the reasons previously listed. I only meant to state why I believe it would be possible for WDW to use plastic APs. I understand why they don't, but I also know firsthand, some of the hardships things like park hopper tickets, APs and SPs all go through. I find it more inconvenient to the guest to have to have it replaced because they got caught in an unexpected rainstorm than to take what may be a few extra seconds to print them on sturdier paper.