Yep, you should try sitting in a remote office, working 10+ hours per day, 5+ days per week, on only your 13 inch laptop, because the client does not provide computers or displays to contractors. Remember that you are doing large graphic work (think multiple 4 ft by 6ft printed presentations) accompanied by large scale numerical analysis.
It does not matter what the computer costs, it only matters that it has ports for use in an office (VGA for the old projectors, HDMI for the new projectors, ethernet (because client public WiFi typically sucks), USB), mag-safe (because there might be 4 to 6 people at one fold up table), large screen, and lots of memory (more than 16G).