Because the swim is so short, it's unavoidable that people will be bunched up in the start of the bike. And then the bike is not long enough to spread out the packs.
In recreational races, the race organizers generally prefer to forbid drafting as a safety measure. In fact, their insurers probably mandate it. It's just dang dangerous to have a bunch of newbies trying to draft in an olympic distance race. For the pros, this is not so much of a concern.