Maybe part tradition and part guidance. I have typically run speakers with twice the power their rating recommended. Not going for loud but impactful.
I think the guidance is to give a very uninformed consumer something to hang their hat on. So, as an uniformed consumer you buy a cheap $200 amp with 20 wpc and try to make these power guzzling towers sound good… they have given guidance that this was not going to turn out well.
Informed audio folks generally ignore most all of the published stuff… or at least look for anything odd… then look at efficiency and listen to what they sound like. I don;t think I payed attention to power handling in any speaker I have bought in 40 years at least.