They have the ability to triangulate the position as independent companies do like www.traceamobile.co.uk and www.mobilelocators.com do? Surely this would be really useful or do they want people to lose their mobile and buy another one!

You just answered your own question lol! Carriers know that more phones are reported for being stolen/lost more than any other reason. They would surely lose out on the $ if they allowed cell phones to be tracked when this happens.

because then you wouldn't have to buy a new one!!!

because they are happy when you have to buy a replacement phone