Because they can't do that for a "good consumer price".
If you want more than ~48GB, you're looking at HBM which is extremely expensive (HBM chips are very expensive, packaging+interposer is extremely expensive, designing and producing a new GPU is expensive).
Normal GPUs are limited by both their bus width (wider bus = more pins = harder to design, more expensive to produce, and increases power consumption), and GDDR6(x) (which maxes out at 2GB/chip currently), so on a 384bit bus (4090/7900xtx, don't expect anyone to make a 512bit busses anymore) you need 12x2GB (GDDR6 uses 32 pins per package) which gives you 24GB. You can double the memory capacity to 48GB, but that requires putting the chips on the back of the GPU which leads to a bunch of cooling issues (and GDDR6 is expensive).
Of course, even if they did all that they're selling expensive GPUs to a small niche market and cannibalizing sales of their own high end products (and even if AMD somehow managed to magic up a 128GB gpu for $700 people still wouldn't buy it because so much of the ML software is CUDA only).
If you want more than ~48GB, you're looking at HBM which is extremely expensive (HBM chips are very expensive, packaging+interposer is extremely expensive, designing and producing a new GPU is expensive).
Normal GPUs are limited by both their bus width (wider bus = more pins = harder to design, more expensive to produce, and increases power consumption), and GDDR6(x) (which maxes out at 2GB/chip currently), so on a 384bit bus (4090/7900xtx, don't expect anyone to make a 512bit busses anymore) you need 12x2GB (GDDR6 uses 32 pins per package) which gives you 24GB. You can double the memory capacity to 48GB, but that requires putting the chips on the back of the GPU which leads to a bunch of cooling issues (and GDDR6 is expensive).
Of course, even if they did all that they're selling expensive GPUs to a small niche market and cannibalizing sales of their own high end products (and even if AMD somehow managed to magic up a 128GB gpu for $700 people still wouldn't buy it because so much of the ML software is CUDA only).