But from what I've noticed, most people don't keep track of how much money they have, how much they're spending and how much they need...
For a person who earns monthly wage, it could be a yes. Yeah! They wrote their weekly budget but do you think it will be done according to the plan? I don't think so. Simply because that person is confident already that he will earn again so overspending won't matter for him as long as he is not exceeding his limits. This is where being irresponsible begins 
.I understand that in order to save money, it is better for me to shop with my wife, who knows all the prices of the products that interest her.
What!? How on earth? What I thought is that women are shopaholics in nature (just in different levels lol). That's why I hate my girlfriend to go with her in the mall unless I'm prepared for squezzing my pocket 
. You've got a practical partner dude. I think those two guys are the women of their relationships... I actually couldn't find from a mainstream search any article where women spend less than guys (although a lot of their clothes are unnecessarily expensive and without pockets - still not exactly hard to add if they wanted to)...
And yeah I might have gone a bit too far on my point with not understanding what you're spending. Since most people know to try to keep within a certain budget, issues arise from things like not actively monitoring joint accounts and other stuff though I imagine for couples...