Zen is way different. Zen does not use a huge complex monolith for their consumer line and is cheap to build. AMD just been delivering at a consistence cadence for their CPU division while keeping power consumption relatively low which is what help it gain consumer confidence. It was not the threadrippers that did it.
Going Halo for Nvidia makes more sense since they have a professional visualization market which provides guaranteed income at high margins.
AMD doesn't have much for this market. As we can see with the possible performance flop of the RTX 5090, going big carries more risk as more things can go wrong. The GTX 480 and Fury X are examples of this. The RTX 5090 is clocked awfully low for the amount of power it uses. This might be a fermi like chip which misses targets in the gaming segments but recovers sales in the professional markets.
Also chip design is more expensive than that. Both companies R and D expenditures seem to line up with this chart.
Here is a newer chart.
I was looking into the software part and it's ridiculously expensive. Someone on reddit says the licenses for single software can be a million dollars per year and indeed use the software mentioned by XPEA mentioned(Cadence and synopsys) and they have hundreds of these licences which are only good for one year. A single big chip can use hundreds of licenses in their design.
So would AMD rather uses these resources on something like instinct or a Halo discrete graphics card? I think we all know the answer. AMD is just following the money.
Spending hundreds of millions on a single halo just doesn't make sense when that money can be spent on datacenter where sales are in billions, not hundreds of millions.