Why? They're not that great.

Gibson doesn't make the end-be-all of pickups. They're expensive for what they are.

You can get comparable pickups for less, or better ones for the same price.

Vintage Original Gorilla Fists from rockmonkeyguitars.com are better, and they're cheaper right now. With the pound weaker, they only cost 100 bucks. Alnico II magnet, as well, so they're around the same tonal range.
Also an option would be Seymour Duncan Alnico II Pro's. If you want that PAF tone, but brighter, go for Seymour Duncan SH-1 '59's.

If you like the 49xx series, just look at a Seymour Duncan SH-1 '59/SH-14 Custom 5 combo. Alternatively, a set of RMG Silverback pickups would be great, too.