The problem is that "daytrade buying power" depends on exactly what you are trading, the time of day, etc. If they had a field that was called "DTBP" that showed 4xEMV, it raises the prospect of complaints from people that get liquidated at 15:50 because of the 50% overnight rule, people that try to buy that much of a 100% margin requirement (i.e. "non-marginable") stock, etc.
Of course, they could just put a field called "4xEMV" on the screen.