TSMC is the only proven fab at this point. Samsung is lagging and current emerging tech isn't meeting expectations. Intel might be back in the game with their next gen but it's still to be proven and they aren't scaled up to production levels yet.

And the differences between the different fabs means that designing a chip to be made at more than one would be almost like designing entirely different chips for each fab. Not only are the gates themselves different dimensions (and require a different layout) but they also have different performance and power profiles, so even if two chips are logically the same and they could trade area efficiency for more consistent higher level layout (like think two buildings with the same footprint but different room layouts), they'd need different setups for things like buffers and repeaters. And even if they do design the same logical chip for both fabs, they'd end up being different products in the end.

And with TSMC leading not just performance but also yields, the lower end chips might not even be cheaper to produce.

Also, each fab requires NDAs and such and it could even be a case where signing one NDA disqualifies you from signing another, so they might require entirely different teams to do the NDA-requiring work rather than being able to have some overlap for similar work.

Not that I disagree with your sentiment overall, it's just a gamble. Like what if one company goes with Samsung for one SKU and their competition goes with TSMC for the competing SKU and they end up with a whole bunch of inventory that no one wants because the performance gap is bigger than the price gap making waiting for stock the no brainer choice?

But if Intel or Samsung do catch up to TSMC in at least some of the metrics, that could change.