What's the trouble with Green!

Why does everyone, especially in watercolour, seem to feel that the ready mixed greens that you can buy are not ok to use straight and unmixed? I find this very odd as I frequently use a green that comes straight from the tube - Winsor & Newton's Olive Green being a good example as it is a very true to nature and lifelike green. Every time I see artists on the TV, etc mixing their greens they often turn out these muddy khaki greens that I personally find very unattractive. Is this a form of artistic 'snobbery' or is there a genuine reason people have for always having to mix their greens? On a similar note, why do most watercolour artists that I have seen painting always mix all their colours. There is such a huge range of pigments available that it is almost unnecessary to do this. I personally do very little mixing, preferring to use pure pigment as much as I can. I find it hard to believe that you just can't achieve the colours you need for a painting with mixing them, as there is practically every colour in the book on offer ready made. Is it one of these things that has grown and is now set in concrete without anyone really knowing why they do it? Or is it something else?