I used to wonder that too, until I actually read good biblical historians. As it happens, the writers of the Jewish Bible were not monotheists for the most part. Early on, they believed multiple gods existed and they worshiped more than a few of them. This is why there is the story of the golden calf, representing Ba’al. If the people didn’t worship Ba’al, why would they have suddenly started at Horeb? Well, they wouldn’t.
In any case, when archeologists dig in ancient Israelite and Judean cities, they regularly - almost always - find household figures of various gods, especially gods related to fertility and weather. Worshiping these gods helped ensure that you’d be able to have children and that the crops would not fail. Ba’al, for example, was considered a storm god (as happens so was Yahweh) and he was also associated with fertility.
At some early point in history, Isreal primarily worshiped El, the father of Yahweh, and the 70 or so brothers and sisters of Yahweh, and then later in biblical writings Yahweh asserted that he was El. But these were two different gods. It’s all in the bible, but hard to see in English. Ever wonder what El Shaddai means? (It means El of the Mountains). Or the name Isreal (Isra-El). Or Samuel?
Yes, it presupposes other gods. If there were no other gods, Yahweh would have no reason to be a jealous god, or to announce - specifically to the Hebrews - that he was their primary god.
Instead, you would think the writer would have had Yahweh saying something like, “I am the only real god. Worship of other gods is futile and has no meaning.”
These things would likely be much easier for us to grasp if the biblical translators had not changed the names of the primary Hebrew gods found in scripture, Yahweh and El / Elohiym, to Lord God and God.
As with all of the other gods mentioned in the bible, the biblical gods of the Hebrews, and later the Jews (and Jesus) have names - and they aren’t “god”. I think it is a huge dis-service to mask that fact in translation.
For example,
Genesis 1:1 somewhat literally reads:
First, Elohiym [a personal name which also means ‘god’] created the sky and the land. And the land was formless and empty and darkness was over the front of the abyss. And the breath of Elohiym was hovering over the front of the waters.
Or even more mechanically/literally, the original words say:
In the summit Elohiym shaped the skies and the land, and the land had existed in confusion and was unfilled, and darkness was upon the face of the deep water and the wind of Elohiym was fluttering upon the face of the waters.
And then…
Genesis 2:4 begins the second account of creation. It somewhat literally reads:
These are the generations the sky and the land, when they were created, in the day that Yahweh the elohiym [the god] made the land and the sky, and before any plant of the field was in the earth, and before any herb or the field had grown, for Yahweh had not caused it to rain on the earth, and there was no mankind to cultivate the ground. Then a mist went up from the earth and watered the whole face of the ground.
Or, again, even more literally:
These are the birthings of the skies and the land in their being shaped in the day Yahweh the elohiym made land and skies, and before all the shrubs of the field existing in the land, and before all the herbs of the field springing up, given that Yahweh the elohiym did not make it precipitate upon the land and it was without a human to serve the ground, and a mist will go up from the land and he made all the face of the ground drink,