AMD has to choose a process years before that process goes into production, so there's a lot of risk that production could get delayed or yields could be poor. AMD could choose TSMC then discover later that Samsung would have been better.
I don't know that it was a lack of money so much as the cost/benefit ratio.
We like to think of all the big chips from Intel, AMD, Nvidia, etc or of bleeding edge mobile processors from Apple or Qualcomm. The reality is that these are a small fraction of the total market for chips. The vast majority of chips are made at 28nm or larger because the cost of production is so much lower.
Supply and demand is another big factor. As supply increases, prices will drop down to bare minimums and if costs can't be reduced enough, there will be expensive fabs sitting around doing nothing. Even if GF has enough money to design and make the fab, that doesn't mean the demand is high enough for them to charge enough to recover their costs.
Instead of spending that money on 7nm, they can spend it refining their 22, 14, and 12nm lines to improve efficiency and lower production costs. With TSMC or Samsung pouring those billions into 7 and 5nm, there's a good chance at GF pulling quite a bit ahead in the slightly older nodes. Once they have done this, those bulk chip makers using 28nm will start moving to smaller nodes. There's quite a tidy sum of profit to be reaped here. Given how every consumer has dozens to hundreds of these small chips for every cutting-edge chip in their home, appliances, or car, there is cumulatively a huge savings to be had in product and energy costs.
They had already developed the node, or in any case, IBM had (and they purchased the IBM fab research division). But they didn't have the capital to scale out. They decided it would be more profitable and less risky to stay on 14nm and serve the long tail of customers that don't use leading edge nodes.
