They don’t get their absurd profit margins by cannibalising their data centre chips.

This is like Intel and their refusal to support ECC memory; when AMD does on nearly all Ryzens.

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Note: your laptop is probably using a 64-bit memory bus for system RAM. For GPUs, the 4090 is 384-bit. That takes up a lot more die area for the bus and memory controller.

But GP's laptop with 96GB of unified memory would be a M2 Max Macbook or better. The M2 Max has a 4 x 128-bit memory bus (410GB/s) and the M2 Ultra is 8 x 128bit (819GB/s), versus a 4090 at 1008GB/s. But see here for caveats about Mac bandwidth: https://news.ycombinator.com/item?id=38811290

Why would they do that with a gaming card? If you want more you can rent in Aws etc.

It wouldn’t be a local model if it has to work on AWS.

Isn't there the risk that if they give the gaming cards enough RAM for such tasks then they'll get bought up for that purpose and the second-hand price will go even higher?

I guess my point is, rather than give the cards more RAM, the gaming cards should just be priced cheaper.

Which laptop models share system RAM with an Nvidia RTX cards?

Op probably referring to an M series MacBook since it has a unified memory architecture and the same memory space used by both cpu and gpu