This is what it's like for me:
You have an idea for something that seems like it would make a pretty funny sitcom. A couple of strong characters who are bound together but don't get on. An easy-to-grasp situation that will support at least a good dozen storylines.
You plot the pilot episode. I do this pretty loosely; I know what the situation is at the beginning, I know what each character wants, and I know what has to happen to move the story forward. I kind of know how it's likely to end, but that's not set in stone. It's written on cardboard. Or virtual cardboard.
You make rough notes on what information need to be revealed in each scene. You try to keep exposition to the minimum, and allow lots of space for gags.
You give yourself a pretend deadline and work out how many pages you need to write per day, and you start writing the dialogue.
And suddenly, you lose control. Some minor character you considered necessary for expositional purposes is getting lots of funny lines. Your main character turns out to be far more obnoxious and confrontational than you'd anticipated. His daughter is pregnant all of a sudden. Some other minor characters exit and then turn up again without warning. The storyline seems implausible, and you wonder why you ever thought it made sense.
Your already too loosely-planned plot structure is now a flurry of paper napkins, blowing about in all directions. You grab at them and try to put them back into a sensible order, but the window's open and it's gusty.
You realise it's going to take at least a day to get the window closed.
Sometimes it's better that the deadline is only pretend.