Becuase I don't desperatly WANT to and therefor I know it isn't for me. Whenever I think of marriage I think of the dress, which means I probably just want a nice frock and a party. Well I can do that without marriage.
Emotionally I feel more free without marriage (I think it makes my man work harder becaus I could walk if he really gets up my nose with no negative effects for me, see below for those of you who will say "what about the house, kiddy custody etc").
I love and trust dp, he loves and trusts me. We ain't going nowhere.
The other things like house ownership, division of money etc I can sort for myself with a good will and a confidence in my own ability to stand on my own two feet should it all go shit shaped. I earn just as much as him, the house is in my name, I have a rock solid will, life insurance etc etc.
These days I thinks that the ability to pre-empt disaster (death divorce separation) is as much down to good future planning as anything else.
I know my situation is a little unique as not eveyone can be in an egalitarian realtionship (moneywise) so appreciate that marriage is best for some and am happy to celebrate it as a good thing.
It is not for me though.