It takes time because the cards are not made onsite by Disney. They are processed through an independent company, and if it is anything like the company a lot of Video stores used to use for their membership cards it workes like this:
1) there is a minimum number of xxx (I think it was 400 for the video store I worked at).
2) The customers names and info are placed onto electonic media (most likely CD-R using todays technology, back then it was tape)
3) The media is shipped to the processing center, who then can take their own sweet time making the cards and shipping them.
4) Disney DDE would receive confirmation after a batch of cards are shipped
It is a time consuming process. DVC cards also take a while to process. No doubt, Disney deems this option to be more economical than buying the equipment to make them onsite...though I don't know why they couldn't use a system similar to what is used for room keys.