lots of parkrun courses involve loops (makes sense really because how many local parks have 5 kilometres of empty space) so it wouldn't work, the arseholes would still end up barging the slower people out of the way on their second loop while the slower were finishing their first (and then third and fourth etc).
Besides which how would you define competitive vs fun? Lots of competitive people are not complete arseholes and can share the course appropriately. Would it be fair for competitive women, who will still usually be slowly than a man at the same level, to be treated badly? And most fun runners are still competitive to a degree, even if just against themselves (i.e improving their own times) otherwise they wouldn't bother going to an event or logging their times at all.
Not to mention PR is all done by volunteers offering their time for free - people who would probably not prefer not to have to deal with extra faff and additional time of organising multiple races and aggression from the idiots.
And more races = longer time other park users can't use the area (or risk getting shouted at/barged out of the way if they do) to play games, take the kids out, walk the dog, use the field for sports matches etc., which given the fact they take place on a weekend morning (the busiest time for most parks) would be a huge issue!
Why should the 99% have to make changes and put themselves out just for the 1% who can't behave properly?
The London marathon is a huge event, with a lot of financial backing, plus support from the council, police, that takes place once a year, in the same place. Not thousands of events taking place in different areas every single week run completely by volunteers. It's completely different
With the greatest respect, if you clearly have never been to a park run, don't you think it's a bit arrogant to chime in with how you 'can't work out' how it could be better run? Without thinking the people who have been organising it for over a decade at this point might have magically thought about these things themselves if it were possible?