It's because reading and writing happen in separate processes, i.e. simultaneously instead of interleaved.

In general writing to disk is handled asynchronously by the kernel (`write` just copies to a buffer and returns), but metadata operations like creating files are not, so this should help the most for many small files.

There's probably more than one reason it's faster. For example, tar ignores extended file attributes by default. Cp -r would have to check them for every file.