Q:

Why do airlines not have you put your bag underneath your own seat?

On airplanes, they tell you to put your bags either in the overhead or under the seat in front of you. Considering that the front row doesn't have a seat in front of them, and the back seats don't have anything under them, would it not be more efficient to have people put their backs underneath their own seats?
Please don't give me an answer about legroom. We all know airlines don't care about our legroom. 

A:

I can think of two reasons.  One, sadly, you have disallowed, which is the legroom answer. But what if the answer actually is about legroom?  Since I don't have enough room for my knees, I have to straighten my legs, which requires me to put my feet under the seat in front of me. It seems to me that if passengers were supposed to put bags under their own seat, people would be fighting over that space more than they already are. 
The other reason is safety.  That space has a bar to keep the things in there from sliding forward in a crash.  This serves as a barrier between the space and the occupant of the seat above.  If the bar were at the back of the seat instead of the front, it would allow things to fly into the aisle between the rows, injuring passengers' feet or impeding their egress.   

A:

The gymnastics required to put your bag behind your feet in such a small space would be very interesting. Bending forward and sliding it in, sure that works. But pushing it back when you can't see and your legs are in the way? Most people are not good at bending into pretzels in 31" of space.

A:

Inconveniencing yourself never creates an argument. Inconveniencing others, does however. If you were to put your bag under your seat, you would be affecting the comfort of the passenger behind you. That passenger may not even have brought a bag on board, but now finds their comfort dependent on the whims of the passenger in front of them.
I think it's easy to see what the result of this is: complaints, arguments, fights, passengers being thrown off the flight. This does nobody any good.

