Terrified of litigation (and quite rightly).
Organisers are simply covering themselves against claims.
And of course most organsiers do take their signage down. One organiser I know offers a free ride in any of his events if any cyclist finds any signage from a past event. He’s never had to give that free ride yet!
I`m not a fan of sportives but putting that aside I dont see why the organizers need to put warning signs up at junctions telling you to slow down or stop etc when there are official roadsigns there as well .These signs will then get left up for months and litter the countryside with neon yellow