cooper actions

The lack of after market parts is prob the main reason. I have owned a few over the years and they are a very nice 3 lug action with a good trigger. By the way why would you ever want to take a Cooper out of that beautiful wood and put it into something else? I think the Colt/Coopers are a good idea but way over priced.
 
why do you not see any customs built on cooper actions

I would say mostly price and parts/aftermarket support. If you're going to buy a gun just to take it apart and modify/replace most of it, why start with something expensive? Also, they do not sell just the receiver (at least last time I asked them), so if you're going to build a custom from scratch it still makes more sense to get a receiver from BAT, Pierce, etc. and most of those are rem clones so you can buy triggers, stocks, etc. that will more or less bolt up. Also, coopers are not very common. I have never seen one in a local shop or gun show. I own one, and I think it's still the only one I've actually touched.

We may see more of them now that colt is using their receivers. Hopefully the quality holds.
 
I would say price versus options. There are to many quality actions out there to buy a $2000.00 dollar rifle for just the action. Plus the aftermarket is not as big for them as Rem clones too. I have one and love it but outside of replacing the barrel, I would not use it since I can get a custom action for cheaper and get all I want for just a little more than the cost of a Cooper Rifle.
 
The lack of after market parts is prob the main reason. I have owned a few over the years and they are a very nice 3 lug action with a good trigger. By the way why would you ever want to take a Cooper out of that beautiful wood and put it into something else? I think the Colt/Coopers are a good idea but way over priced.

This sums it up nicely. If you want a great shooting, great looking factory rifle it is hard to beat a cooper.