The reasoning behind modern slow play as taught at the elite level is circular.
One tenant of their belief system is that the first imperative, the #1 priority, is to completely reset your preparation for a shot to the very beginning if anything causes you to deviate from doing it in the usual way, if anything distracts or catches your attention or if anything within your field of view or hearing changes while you are engaged in "preparation".
Well given that, of course you can't start while someone else is hitting. Because that would mean doing your routine while somebody within eye or earshot of you is doing their thing. The potential for so-called distraction is oh so great.
This really becomes a problem when they extend their preparation further and further before the actual shot. I learned some of this from a now leading practitioner of golf psychology a good many years ago now. At that time it was also preached that you can be doing all the assessment of the lie and wind, choosing a club, practicing swings, anything except visualizing the shot, stepping up and hitting it prior to your "routine". Over time it appears to have evolved to where all those elements (damned near everything except walking from the tee to the fairway) is part of a single can-not-be-interrupted procedure.
Heck, I remember doing drills with a stopwatch. If I had not struck the ball within 16 seconds of reaching for the club in my golf bag the coach would say "Too Long". The whole idea was to make your "routine" something simple, committed, quick and continuous. Not sure when that definition got changed.