The cheapest available Epyc (7313P) has 16 cores and dual socket systems have up to 128 cores and 256 threads. Server workloads are massively parallel, so a 4+4 core M1 would be embarrassed and Apple wouldn't want to subject themselves to that comparison.
But another reason they won't do it is that TSMC has a finite amount of 5nm fab capacity. They can't make more of the chips than they already do.
A 4+4 core M1 is 16 billion transistors. Some of that is the little cores, GPU, etc, but its not clear to me its practical to get, say 8x larger. That would be 128 billion transistors. As a point of comparison, NVIDIA's RTX 3090 is 28B transistors, and thats a huge, expensive chip.
But another reason they won't do it is that TSMC has a finite amount of 5nm fab capacity. They can't make more of the chips than they already do.