I will go with M3Boy on this one.
The EPT/WPT is filled with alot of amatuers, (no they aren"t all THAT good) but is also well stocked with professionals. This is THEIR bread and butter. They play the tourneys week in/week out. This means that they are used to the "big stage" and are probably less intimidated by the consequences of being knocked out (ie it won"t drastically effect their roll). But more importantly, as M3 Boy quoted, I believe that they have learnt to adapt their game styles so that they can cope with the differing styles they encounter.
I have said it before, but I was absolutely in awe of Henning Gransted at the EPT. His reads, lack of tells and ability to change gear were really impressive. I believe to get that good you need to be on the circuit week in week out.
I don"t beleive its entirely impossible to win one of these as an amatuer - just look at the number of online qualifiers that hvae won them. But if you ever get there you need to forget the big occasion and play your "A game". But even that could be tricky. I sincerely beleive that you need to learn a few different styles to truely be prepared for one of the big tourneys.
I"ve actually spent a few months denting my bankroll trying and trying to play different styles in tourneys. I"ve now started to blend them and the results really are starting to pay dividends live (though I blew it this weekend!!!). But all that change has been brought about from what I saw in Dortmund, where I realised that if I were to progress then a change would be needed. I think its been worth it and a much better player than even a few months ago. Time will tell...
On the subject of tourneys, I think that the GUKPT tie up is a more natural step for an APAT player. You get to play in your own country against (mostly) Brit players. Its not as far removed from the comfort zone as playing in a foreign city, with crazy foreign players.
Oh and of course you need to Poker God to be on your side. So next time an APAT player goes into the EPT we all need to hold a vigil to Poker God and offer up a sacrifice!!!!
That duck springs to mind Phil ;-)