Because boobies fix EVERYTHING. Why give up such a fantastic, reliable parenting tool?
My daughter self-weaned just before the age of 3, because I had started to misuse that awesome parenting tool to get her to nap in the afternoons ;) She's a bright little thing and caught on to the fact that when it was about 2 pm and I said "Let's go have cuddles and boobies in the bedroom" it meant that she would inevitably fall asleep. Because I was going through a clueless phase I didn't get that the fact she was awake until 11 pm meant that she was totally ready to give up her afternoon nap - but she knew, and she started saying "no" to the nursing offer. That was her last regular nursing session - bedtime was cuddle-time with Daddy - so when she stopped the naps, she stopped nursing. A few weeks later she wanted to nurse again and she tried but there was no milk. I told her she could probably get it back if she was willing to nurse every two hours for a few days. She thought about it for about 2 seconds and said "No thanks" and that was that.
But prior to that, nursing got us through her bangs and bonks from walking at 10 months, her pre-verbal frustrations, travel, owies, sadness, boredom - everything. She's six now and sometimes I still miss it!