In this case, because the storage device that's there by default is optimized for low power and not high speed, so RAM is dramatically faster. They implemented swap as well but only as a last resort when the compressed RAM runs out.
"Zswap is a kernel feature that provides a compressed RAM cache for swap pages. Pages which would otherwise be swapped out to disk are instead compressed and stored into a memory pool in RAM. Once the pool is full or the RAM is exhausted, the least recently used (LRU) page is decompressed and written to disk, as if it had not been intercepted." https://wiki.archlinux.org/index.php/Zswap