Short answer, they cant be made any more.

The eldar just dont have the resources (choirs and spirit stones to name a few) to be able to build an entire craftworl. Remember that these things are essentially moving planeta for the eldar, with some have billions. Also remember that to get spirit stones to build an infinity circuit one must go to the warp and battle slaneesh on his/her planet to ge them. The trade is usually to high just a o get a small amount otherwise the Eldar would be raiding the Eye of Terror all the time.

Also i cant imagine a scenario where the Eldar have enough folk or time to just drop what they are doing and declare allegiance to an entirely new Craftworld.

Finally, why? The craftworlds were designed to be moving pleasure worlds during the Eldar's height of power before the fall. The only reason they are used is they survived. If the eldar somehow got the singers and the population and the Spirit Stones AND time to build it, they would probably design something more purpose built for the current statw of the galaxy.

Much like the most baddas things in the imperium, the Craftworlds leftovers thst happened to become lifeboats for the Eldar.