Interestingly. I was watching some interviews with the founders of 3DFX, and when asked about why the company died, their straight answer was: because they bought a fab.
I thought it was because 3DFX started to compete with their own hardware partners?
3dfx was a chipset vendor - whether or not they fab their own chips shouldn't matter all-that-much: it's who makes and sells the boards that counts. The Voodoo1 and early Voodoo2 cards were made, packaged, and sold by hardware partners like Creative, Diamond, etc - but with the Voodoo3 and later SKUs of the Voodoo2, 3dfx did it all by themselves, so why would the miffed Creative Labs and Diamond lend their sales channel expertise to 3dfx? PC OEMs like Dell, HP, etc also probably had deals with Creative for their Sound Blaster, and it wouldn't surprise me if Creative politely asked them to not buy 3dfx-made boards in exchange for a sweet discount on Sound Blaster cards...
Yes, exactly this, they bought STB Systems and began manufacturing their own boards. I wasn't sure how much of the manufacturing process this entailed.