Probably the manufacturers of the cans who noticed that the bottoms, with no re-enforcement were stronger if they were roundedThey are more concerned with presenting their product than keeping dumpster-divers happy.
I agree with what both of the people above said but to add to that it makes them easier to smash them yourself into a smaller shape.
They aren't getting stronger, you are getting weaker
Get a big bag of balloons, blow them up and fill their bedroomWrap everything in their bedroom in wrapping paper, or aluminum foilStretch plastic wrap underneath the toilet seat across the porcelain.
