It's not that Mac hardware is only available with limited memory and storage (you can get a MacBook Pro with 128 GB of RAM and 8 TB of storage, or you can get a Mac Pro and throw in a bunch of M.2 drives on PCIe cards), it's just that the pricing on them is ridiculous.
It's not a problem of macOS in the strictest sense but we're talking about highly vertically integrated platforms. You don't get macOS without a mac, so it _is_ a macOS problem in that sense.