"Why not go joint venture with some of these plants to expand capacity"
Several reasons. Remember when AMD tried to fab at UMC? Now there are all sort of rumors as to what the problems were, but I will point out that the fastest complex device that UMC or TSMC (can't remember who fabs for Via offhand) produces is a Via processor at 1.4GHz.
About the only companies that possibly could fab a bleeding edge processor for AMD is Intel, IBM and possibly Motorola. Motorola is trying to dump their fabs, no real long term possibilities there. IBM might be able to do it, but they might not. As far as Intel...
For better or worse, it takes a process tuned for high speed and tight intergration between the process and the design to get a modern, high speed processor. When Intel chose to push the performance envelope with the Pentium Pro and its descendants, the handwriting was on the wall for fabless processor companies. That is why they died.