The assumption is that someone may be doing their first FTP test, so it would be rather circular to tell them to do 5 minutes at something like 130% FTP. Saying "all out" and stuff like that is probably more useful at that point.

All the same, I'll join the chorus of people who find that part of the protocol problematic. In general if a person has done a variety of hard riding efforts over a range of durations of 90 minutes or less a pretty convincing power curve emerges. I personally know from trying the official test protocol that the all out 5 minute effort disrupts considerably my 20 minute power ability. The result is a FTP test that shows me scoring considerably lower than other objective measures.