The thing is no one exactly knows how long the campaign they plan to transfer would be running. Some really do prefer to find a higher-paying campaign due to their own reasons. Personally, I don't do this as I also want to be part of a long-term campaign.
But yes, that's also a hassle for campaign managers to find a replacement participant for the campaign as they need to filter all the applying participants and find the best fit for the position.
Finding the participant to fill the open slot will not be a big task for the manager since people will be ready to rain applications even if it's one free slots and who says no to money. 
You are right and I completely agree with you, from my personally observations, for the campaign managers to find a replacement for a user or some users who left the campaign and created open slot(s) isn't always that much of a hassle compared to when the campaign starts newly and the manager have to select the best participants for the campaign, I personally believe that to fill an or open slots, the manager already knows who and who to accept their application even before he or she declares the campaign open to accepting new applicant(s).
But some members may tend to have it as habit of switching one from another and if they accepted this week and participant decided to switch in the next week may create bad impression and will not likely to be accepted in the future.
Well, also very correct, some users have that habit of shifting from one campaign to another which most of the time is either..
1. For a higher paying campaign.
2. Or for a campaign where they find the campaign requirement more comfortable and convinient for them.
3. Or just for fun.
As in number 3, some users switch between campaigns just for fun, but participants only do this when there is an abundance of campaigns with open slots on the forum, not when the forum is running shot of campaigns with open slots.