I think I've figured out my issue with 'just write' being advised to readily:
You can't "just write" until you've made a decision about what to write, even if it's just the next sentence.
On one extreme, complete pansters make decisions about what to write sentence by sentence, paragraph by paragraph, scene by scene.
On the other extreme, complete plotters make as many decisions as possible before they start writing. When they do start writing, they're only making small scale decisions, like how to phrase dialogue or describe specific actions.
I'm guessing you're not a pantser, because, no.
I sometimes write as a complete pantser, but I write with the whole book in mind all the same.
As Buffysquirrel says, no. I'm a pantser. Not only am I writing with the whole book in mind, but as I write I'm making word choices only, pretty much. I'm usually not consciously making story decisions with every word or sentence. I'm just letting it flow. If anything I'm NOT thinking, more than thinking of everything in minute details.
I'm making the exact same decisions, as a pantser, as the ones you've described for your complete plotters. Exactly the same.
The decisions about where to go next come after I've finished a particular scene, not during it.
No offense, but if you haven't done it and don't understand the process, don't make blanket statements about how it works.
And those complete plotters who make all the decisions ahead of time still have to
make those decisions,
write them, and see whether or not they worked. Having a perfectly plotted outline doesn't mean the book will work once written. Nor does it mean the book will be any good. Nor does it mean the book won't need editing.
You won't know any of that until the book is written.
Telling a plotter to "just write" before they're done with a plot outline isn't just useless advice; it can be harmful advice. You can't just make the assumption that everyone's process is the same as your own, and anyone putting off writing is doing it for no good reason.
How is it harmful? Because they try to write it and find they're stuck and need to work more on their outline? Is that harmful or helpful? Or maybe because they learn they can stray from their outline? Or maybe they learn they have a hard time pantsing and so
need the outline? All of those are valuable things to know.
Advising someone to JWI isn't the same as assuming their process is the same as your own. It's offering what worked for you. Nobody here advises JWI with the intention of making the OP feel stupid or small. We're not rubbing our hands together with glee at the idea of ruining somebody's book. We're trying to help. We're giving advice we've seen work, that works for us. (Personally, if a whole bunch of people were advising me to do something, I'd try it, thinking perhaps they're right. But that may be just me.) It's free, no-obligation advice, and for that price no one is obligated, either, to spend hours investigating the OP's specific process. (I also point out that there seem to be more pantsers than plotters here; what advice do you expect us to give?)
And again, if you don't like the advice, ignore it. If you try it and it doesn't work for you, say so. Nobody's going to check up to make sure you did what they suggested. Nobody's going to call you names if you try it and it doesn't work.
But the fact remains that even those who use outlines often tell people to just write it. Because at some point you have to.
And because you can't edit unless you have a draft.
And because sometimes just writing will teach you something about the story or kick-start your mind into solving that plot problem/character issue.
And because no matter how good your outline is, you never truly know if that will translate to "good book" until the book is actually written.