Because it doesn't make any sense.
With this theory it's basically saying that Dooku the villain gives Jango, another villain, a little boy as a present.
I can't see why Jango would want a kid hanging around. And what's so special about the first clone? There are going to be millions of Jango look a likes going around so it's not as if Jango wanted a replica of himself. There is going to be loads of them anyway.
When Jango dies there are going to be armies made up of his genes. Armies of Jango Fett's. So why does he want one as a present?
At the moment these are all just theories and no one actually knows what is going to happen until next year when the movie comes out.
To say that Boba is a clone of Jango...nope, dont like that idea at all.
And, a reason for the Stormtroopers being crap shots being that they were raised in a lab rather than with a family? Pants!
Its getting a bit too "Space: Above & Beyond".
MOTS, most people here know about my cynicism about rumour and speculation.
Bit to dismiss some of these things as 'just' theory is very short-sighted. Painful as it may be to realise, some of the stuff that has been blown about the film is almost certainly true.
Keep an open mind. You don;t have to believe it all, but some of the stuff that has been leaked is, from a combination of leaked storyboards, script extracts and shots of the film that are all complimentary, very likely to be true.
Though as for Stormtroopers being clones... HHHMMM...