They are inefficient and conflated machines - two things that tend to be contradictory with software engineering.
More practically, many people find themselves completely dependent on Apple's design whims.
Mac Pros (or server racks in general) are closer to the kinds of machines that we deploy to (they use Xeons for one thing). They don't overheat so easily.
Their 'sea of cores' approach enables more productive workflows and tooling.
And one gets to use his favorite peripherals, in an ergonomic position (instead of slouching around a laptop).