Some scenes grainy?

[Gains]

I've seen this pretty often and always wondered why some scenes are grainy.
For example Jurassic Park, some scenes are cristal clear and some scenes are grainy.
I'm not complaining since I knew about this beforehand, but why are some scenes grainy?
I actually thought that every scene is shot by the same equipment but apparently not hence the grainy scenes.
And also, how can for example Steven Speilberg be satisfied with such bad quality?
And why didn't he re-shoot the scenes?
Or is the graininess "created" when transferred to bluray?

I wonder how bad it will be when transfered to 4K Ultra HD.