Why Do Cars Have Blind Spots?

Just a random thought that popped into my head while driving today. Why is it that cars still have blind spots?

Surely, with all our engineering and technology expertise, we can figure out a way to overcome this simple design flaw. I know there have been some attempts at using sensors to detect other vehicles in blind spots,  but nothing much seems to have come from that.

On the Car Talk web site, Tom and Ray suggest that one can actually line up rear view and side mirrors to have minimal overlap. I tried it, but it still doesn't completely eliminate the blind spot.

Years ago, I remember having spherical add-on mirrors on my pickup truck side mirrors, but they weren't that useful.

Surely, some kind of simple proximity sensor could be designed that would alert us to the presence of vehicles in our blind spots? Or car and mirror design could be re-engineered to eliminate blind spots altogether.

Too expensive? People won't pay the additional cost? Does it really have to be expensive?  I think not.