I ask myself that with every FOIA request. If I had a dollar for every internal email I obtained saying that the public body intended to ignore the FOIA and make me sue for access, I would certainly be able to buy a sandwich.
Why do public employees do this? It's not their data. Why do they fight tooth and nail to avoid responding to FOIA requests for public information? If you want a government employee to do some work, file a FOIA. They'll work their little butts off day and night to do whatever it takes to not send you the requested docs.
There needs to be a formal study done of why this happens.
My guess is that citizen's freedom (of information) places a compulsion (of fulfillment) on the body being summoned, and our natural inclination has always been to resist being compelled.
It may have been a mistake to phrase the FOIA process to the affected bodies as "you must share" rather than "you now have the tools and process to share".
One of my first real interactions with a public body was filing a FOI request to a local council. I'm still a little shocked that they made me fight it all the way to the regulator.
I think it harms the ego of the government workers to think they are effectively of lower status than a random citizen who can ask for them to account for themselves