As is automatically assuming they are right THIS time. As it stands currently none have given any evidence of their claims, just like they didn't when the Euro question came up. They were wrong then and there is nothing to evidence that they will not be wrong this time.
What do you mean no evidence? These are predictions modeled on likely economic impact. That is what top economists say (https://next.ft.com/content/1a86ab36-afbe-11e5-b955-1a1d298b6250), it is what the Cabinet Office say (http://www.theguardian.com/politics...ect-lives-of-millions-official-uk-report-says), it is what the Bank of England says (http://www.bankofengland.co.uk/publications/Pages/news/2016/032.aspx).
There is evidence if you care to look. What we don't have evidence from from the Out campaign is how the UK will be a stronger economy outside of the EU. This raises further questions which have not been answered by outers: what will the UK's trading relationship to the EU be after Brexit? in what ways will Brexit impact the UK economy after losing access to the single market and having to trade with all other countries on a WTO basis? how long will this uncertainty last? how long will renegotiation take place? how can UK citizens be insulated from this economic trauma?
It's all based on best-case scenario, leap into the dark, Brexit-and-be-damned sort of stuff. No evidence, just bluster.