Why indeed? It's a valid question. Some people get them just because they can, others value a super dependable work tool and others get them to look cool, which is perfectly fine too. Why not?
But buying a 1DX won't make you a pro photographer, it just makes you a 1DX owner.
There's a raft of reasons people buy 1-Series bodies. They're just rock solid in a way that 5D3 and 6D just don't come near. I shoot daily with 5D3 & 1D-Mk4 and the Mk4 is the winner by orders of magnitude for handling, speed of operation, "just perfect" ergonomics, almost insane levels of weather sealing and long term dependability which is vital in business. The 1D4 has a shutter count of several hundred thousand and hums along like new whereas the 5D3 is now on it's third shutter. It's not an expensive job but equipment failure can cost you your reputation. Previous 1-Series bodies have been retired with 700K+ shutter counts and still functioning perfectly.
The 5D3 was my first non-1 Series body since the original 1Ds. While I love the files I get from the 5D3 in hindsight I wish I'd coughed up for the 1DX. I'll be first in line for the 1DX-II. Once you've travelled first-class it's hard to go back.
-pw