Not true, some have.
https://www.economistsforfreetrade.c...op-economists/
I have no objection to the Treasury, Bank of England, ONS etc producing forecasts - it is there responsibility to do so. What I object to is the interpretation of them as being "facts" when they are anything but. So much depends on the assumptions that are put in to them.
Even less politically motivated forecasts can be blown off course by external events anyway. As already mentioned the Treasury didn't just get their pre-referendum forecast for the next two years slightly wrong, they got it spectacularly wrong. Just as a reminder by now unemployment should have increased by 500,000 to 8000,000. In fact it has dropped by about 240,000. So why should we trust their 15 year forecast?
And this is not just a one-off, the Treasury has a long record of being wrong - ERM and the Euro for example. If the UK didn't join the latter the City of London was apparently going to become a backwater.
As for the Bank of England, even Paul Krugman the left wing American economist who is no fan of Brexit thinks their report is politically motivated.
Ultimately the only way we will ever know if Brexit is going to be a success is if it is given a chance to happen and then we will see. I am now convinced that it is not going to happen at least not in any meaningful sense. The Establishment in the form of the Government, Civil Service, Bank of England, Parliament, BBC etc. are going to make damn sure it doesn't.