It's not "just, because". And it's not anti-development. It's anti taking of open public land, that is not themed, programmed, dedicated to a single use. And making it, for the next 99 years at least, off limits to any other use.
This is much different than constructing say a field house of equal footage in a park. A field house at least can be used by any organization for any purpose. It does not belong to one organization, one theme, one use, and that use only. (Note, I'm not a fan of field houses in parks either, when they are better sited elsewhere).
Now on the subject of roads and parking lots, to be consistent, no, those are not parkland either, they are plazas. Were you aware that the United Center is surrounded by acres of open green fields?
Plazas are not parks. Daley Plaza is not a park. But it is non-themed open space, and I bet everyone here would reject losing Daley Plaza for a building. A parking lot, a street can be closed at any time. Used for any other use like any field.
Columbus Drive is closed and used for street festivals, races, etc. Same with the tailgating lot at Soldier Field. They are closed to cars, and nothing says they have to have cars for the next 99 years. Nothing dedicates them to one sole use. And they are easily converted to parkland open space if that's what a future public decides.
There is a real legal distinction between putting a private organization's private building in a park that is not at all comparable to existing public buildings, roads and parking lots.