Because it started life as Bushy Park Time Trials. Its origins are competitive. Lots of people like that which is why they don’t want to get rid of it, I’m sure participation would drop.
After thinking about it recently, I think they should only send finish times to the individual participants and not publish them, and not use/publish finish positions or age gradings. That way if you want a fun run you get a fun run, but if you want to test yourself then you have a record of your times.
Yes sorry I know that my point was if it’s not meant to be a competition then why do Parkrun also provide age grading etc.