Because it is impossible, unless you create initial parameters.
In a fairly simple project, you are probably more or less safe, but not with a complicated Post-project with tons of automation.
Say, you insert a third party plugin in a track.
No need for automation.
Project done and archived.
Plugin gets an update.
You reopen the project with that updated plugin.
What are the chances that the plugin snaps to the same preset and/or parameters you have initially dialed in?
Open a project which has no automation written.
Accidently (or intentional) you move a fader or a parameter.
How can you know?
Write plugin automation halfway the project (nothing at the start)
Close your project when your cursor is over halfway.
Reopen.
Go to the the start of your project without playing back the project. Just open and go to start.
Press Play: There is a fair chance that your plugin will remember it’s last settings, and not the settings it needs to snap to at the beginning of your project.
Etc … etc …
The only way to guarantee that your project always plays back, just the way you inteded it, is by writing automation parameters from the very start.
Fredo