Because it will take ages (DDs are 2.6 and 7 months) and my house is a sea of toys and trodden in bits of biscuit etc.
I'll spend hours cleaning/tidying, then have to go out all morning tomorrow so DDs don't mess house up before tomorrow afternoon and will end up buying lunch in pizza express to keep us out the house. My jeans will then be even tighter and we will be just a little bit poorer.
If I thought it would all end in an offer then it would be fine, but it will end up with us being told "they wanted something with more bedrooms" or "the garden wasn't right" or my personal favourite "you can see the garage from the study" 
Anyway, if they're buying a 'family home', surely they should see what it will look like when a family live in it, shouldn't they?
DH thinks not, and has started on the tidying...