It's just as much "change is scary" and "I like this as it is." It's a very human reaction.

I dont think its just that. Nimbys also see:

- many new building being very ugly (side note: ugly buildings no matter how green get torn down and are this not as green as building that are beautiful)

- increasing density bringing increased crime

- increased density actually turning out to be less efficient on a per capita tax basis (this is always wild to me, cities should be spending much less per capita than rural areas but arent)