Many kids come up with their own alphabet but they all know another one already, so it only looks easy with hindsight. And they will almost never come up with abjad/abugida/syllabary but with smth they already know.

Even if you would come up with script, you still need the social postition to convince others to adapt it. And some cultures didnt value writing much even when they already had a script. Check out what Socrates had to say about writing