Probably the ones who have 32gb of memory for their photoshop scratch buffer. There's still quite a few of these types of professionals. You just can't put that much memory in the Macbook Pro. Processor speed is probably less of an issue compared with memory capacity.
Because 32GB is the supported max for the lower end Mac Pro. The higher end ones support up to 64GB officially, 64GB and 96GB respectively are the actual maximums.
