That is, monotheistic as in One without another. If there is no “other”, then God cannot be individuated.
Sure it can be. The adhearants to the Abrahamic pantheon do it as a matter of routine.
Your understanding of the three Abrahamic religions is superficial and flawed. At their core, none of those religions postulate an individuated God. Some of their adherents who misunderstand the teachings may falsely believe in an individuated God but the religions do not. Your argument is only with the fundamentalists/literalists who mistakenly think God is an individual. Because you think in terms of God being an individual you too are a fundamentalist.
This may help - because God is everywhere, all the time, God cannot be seen or even understood using linear rationality (because it is mired in time and space). Accordingly, some attempt to apply human relationship models to their RELATIONSHIP with God, i.e. parent-child, master-servant, friend, etc. This could be one source of the mistaken belief that God is an individual.
The other reason is simply ego. Some are so enamored with their abilities to think and create that they mistakenly attribute their form to God the “Creator” of all.