I think too much special attention, rewards, incentives etc end up making the SATs into a much much bigger deal than they need to be, and even children who weren't worried or stressed by them start to think - hang on, if this is so important that I'm getting special breakfasts, extra playtime at school, rewards, doughnuts on Friday, money for doing well, everyone else in the school tiptoeing around the Yr 6s, early bedtimes, extra curriculars stopped in case they make them tired, etc - then maybe it's more important that I thought and I should start worrying and treating it like a massive deal. (Not that everyone does all of those, but certainly some pupils I know have quite a selection from the list).
In contrast, schools that don't do a lot of special stuff, assume they're just like normal days, of course you go to all your other activities, life and lessons carry on, maybe have a bit of a treat on Friday or a little extra playtime each day - children take them in their stride, think of them as challenges and chances to show what they've learned, and forget about them a lot faster. (And don't start to expect that the world stops every time they have exams in the future, either, nor that they will be rewarded at every step of the way for the fact of having to do them).
So incentives don't always have the intended effect, and people don't realise until later how the child saw it (but obviously everyone knows their own children best, and there are some where this might be the only way to get them to do them, etc., so I do understand that there are other circumstances).
(I meet a lot of Yr 6s at extra curricular things, from a variety of schools, so get an idea of how different the approaches can be)