> Why don't they just say "This seems to be a 'brushing scam'.

Because it's more definitive to say we have *no* evidence than possibly to imply, by omission, that we *might* have some, but if we do, we're ignoring it.

Compare:

1. "We have no evidence that alien spacecraft are real."

2. "Alien spacecraft seem not to be real."

The statements are not precisely equivalent - particularly if you're an official spokesperson (or a trial witness), whose statements may be scrutinized for weasel words.