People do not know because no one has told them
Here are some giggles for you:
When I was pregnant with my daughter, my aunt gave us the seat that she had used for her grandson, who was at the time, 12 years old. My mother took it to a check and they confiscated it (thank goodness) and she came home without it. I was livid. I was like, "What do you mean they TOOK AWAY OUR CARSEAT?" She said they said it had been recalled. I said well when the company recalls a product they are supposed to give you a REPLACEMENT!!! I fumed for weeks.
I got a Graco travel system for my shower, with a Snugride. I took out the manual to put it in the car, and I was like, backwards? It goes in backwards? Why on God's green earth would I want to put it in BACKWARDS? I almost took it back.
Oh, how far I have come.
DD recently totally outgrew the MA, and we had a spare one, so I took it to my cousin. She has two kids and babysits and said she would love an extra carseat. She said "I'll be sure and return it to you when you need it." I said, well, it expires in March of 2009, so by the time my baby is born and out if the infant seat, this seat won't have a lot of life left. I'll just buy a new one. You can keep this one until then, but when it expires, I do want my Fido cover back!" She looked at me in TOTAL SHOCK. This is an educated woman, she is a GREAT mother, and she has always shown the utmost care for children's safety, but she looks at me incredulously and goes, "Carseats EXPIRE? I had never HEARD of such a thing." I said yeah, they're only good for six years. She was like you're kidding! I said no, check your manual! Honest!
If I were not an internet junkie, I would probably still be like her! Who reads the manual cover to cover when they get a new product? I always read the carseat ones now, but dd's bike came with a manual and I don't even know what I did with it. Ditto for pretty much every appliance in the house. Carseats LOOK so simple. You put it in the car, you put the seatbelt around it. What's to screw up?
I did not carefully read the manuals for dd's bike helmet, riding helmet, or life jacket. (Although we do have the helmet fits checked by someone--the lead car seat tech in our county also does bike helmets). My point is, I'm quite possibly endangering my own daughter's life in ways that I'm unaware of. I hope not, and I certainly would never do it knowingly, but it's possible. So I try to be understanding.