For me, it not really clear why Ron Swanson is working at the Parks Department:
Ron hates the government.
Ron has a lot of money. (He mentions several times that he has buried gold.)
He doesn't like the job and is not interested in anything related to it.
He has experience in working for other companies. (He mentioned to Leslie that he was employed when he was very small.)
The only reason I see is that Ron wants to cut the budgets from government projects – but in this case: Is it clear that he would directly become the person in charge of the department? And if this would be the only reason, why isn't he applying for city manager or similar?
Mostly, Ron works in the department because of the Rule of Funny. Having a dour anti-government personality is great for Leslie to play off of.
However, Ron also keeps his job because he wants to make the government as inefficient as possible. He doesn't just see high spending as bad but everything the government actually gets done. He keeps his position so he can slow things down and so he can hire inefficient people. This is especially highlighted in his episodes with Tom and April, who he hired because they were inefficient and unmotivated.
While vastra360's answer is correct and it is great for comedy effect, it may also interest you to know that Ron says this in the Pilot episode:
I’ve been quite open about this around the office: I don’t want this
parks department to build any parks, because I don’t believe in
government. I think that all government is a waste of taxpayer money.
My dream is to have the park system privatized and run entirely for
profit by corporations, like Chuck E. Cheese. They have an impeccable
business model. I would rather work for Chuck E. Cheese.
This suggests another reason for his continual work for the department is to stop them from actually achieving anything (unless of course they're privatized, in which case he could become super-productive... doubt it though :))
My theory is that Ron Swanson met former Parks department director Michael Tansley in the early 90s when Tansley was smoking marijuana in all the parks in Pawnee.
Infuriated by this pot-smoking liberal government worker who represented everything he despised, Ron attempted to remove him from office but found the bureaucracy of local government to be impenetrable.
So, he got a job at the parks department and gradually worked his way to the top and stole Tansley's job.