Speed
The main reason is cost. A plasma-cutting process can produce a high level of carcinogenic dust. This dust must be captured in an extraction filtering system. This and the cost to completely cover the machine to evacuate it raise the price significantly beyond what an average fabricator is willing to pay to get a beam coping solution.
Cost is the main reason. The plasma cutting of steel produces a lot of carcinogenic dust which MUST be collected in a dust extraction filtering system. This, along with the additional cost to fully enclose the machine in order to evacuate dust, raises the price far beyond what the average fabricator will pay for a beam coping system.