The largest bags of flour on most supermarket shelves are 5 lbs. As someone who used to work in supermarkets, those bags fit nicely into the assigned spaces.
Let us say the supermarket wants to put in another brand in addition to the original brand. Well, that takes more space.
And if the original brand wants to put out a ten pound bag the folks who run the markets are merely going to say we have no room and let the folks buy two five pound bags.
And so the flour producing folks realize they are not going to make a whole lot of money packaging ten pound packages.
So at the market you find five pound bags. The next jump is to the 25 or 50 lbs at the discount places. There the flour producing folks can make a buck.
Just my take on things.
Before criticizing a person, walk a mile in his shoes - then you are a mile away and you have his shoes!