A pin on underside of CPU is reasonably cheaper especially with how they've got a sea of wires working for the humongous models replacing previous serdes-based approach
The thing they're doing which is completely insane is using different sockets for Threadripper and Epyc.
The primary difference between them isn't the price -- they're both expensive -- it's that Epyc uses the power budget to reach higher core counts and Threadripper uses it to hit higher clock speeds. So for example, Zen5 Threadripper 9975WX has 32 cores with a 4 GHz base clock and 5.4 GHz boost clock. There is no equivalent Zen5 Epyc, the most similar is 9375F which is 3.8GHz/4.8GHz. Conversely, Epyc 9755 has 128 Zen5 cores and Epyc 9965 has 192 Zen5c cores and there is no Threadripper with that many cores.
The reason this matters is that the system boards themselves are pricey, not just the CPUs, but as the platform starts to age you start to get a lot of old servers on the used market that originally had a bottom-end Epyc in them. Which you'd then like to pull and replace with a Threadripper -- i.e. pay them more money to buy a new Threadripper -- except that they inexplicably decided to screw you and themselves by using a different socket.
Perhaps it's better to stick on am5 for a while, but it's not clear if this number of pins is somehow cost effective, or if it's just enough to beat Intel.