so this might be a stupid question because I just started reading the books for the first time, but why didn’t the Lannisters take the throne for themselves instead of giving it over to Robert Baratheon? more men would’ve been loyal to them than to the Baratheons because they had more money, right?

No. Robert Baratheon had the North, the Riverlands, the Vale, the Iron Islands, and the Stormlands behind him. The Lannisters had just alienated Dorne and the Reach, both Targaryen loyalists, by murdering the Targaryens. 

The Lannisters would have been alone against the rest of Westeros. And it doesn’t matter how much money you have if no one’s willing to do business with you.