Why not?

I believe that G3s lacked the necessary processor components (instructions, I don't know) that would allow it to work with another G3. So, it is actually the G3 itself, not the motherboard.

There was a short lived upgrade that Daystar, or XLR8, offered that allowed one to connect two G4 zifs up to a single zif slot. There have been previous topics on this matter.

yea it was pulled it cause of it caused hdd corruption and it dont work in OS X cause it only seen one cpu in the dual carryer card. but if you got the cpu's that was made for it wich they also made it worked better. but still didnt wour fully under OS X. but i think the reason it had so much problems was cause you could pair any speed of G3 with any speed of G3 cpu or any speed G4 with any other speed G4 cpu. say like one cpu was 300mhz and the other cpu was 500mhz it allowed you to pair the 2 togeather witch you would think it would cause a problem cause of the 2 diffrent speed of cpu's. The beige G3's logic board didnt really have the proper hardware to support MP = muti prosseser's. but still is a cool consept tho

It was the ROM that was the majour problem, I think. The management of the two CPUs would have to be partly in the ROM. The G3 also wasn't built for dual processing; it's just not good at sharing information with another processor. As for the Dual G4 thing, they moved to a different processor card form factor with AGP models. IIRC, there were no dual PCI G4s.

The G3 processor has no provisions for maintaining cache coherency. This is required for SMP. You can use them as secondary processors in a DSP array however.