Because economics is complicated and these people just think big number is good, taxes make big number go down, taxes bad. The places interested in splitting are always generating above average funds and are upset that their funds are being taken and used in poorer performing areas. They should just keep all that money to themselves and things would be way better! That's ignoring all the benefits of the collective, but those benefits are often abstract, intangible, and hard to turn into dollar amounts--until the spit happens of course.

Economists predicted the UK leaving the EU would be bad but now they've been able to put a number on it and it's "£100 billion a year" bad. The money saved in EU fees is nowhere near enough to offset the loss of free trade agreements, loss of company headquarters, loss of workers, loss of investments, loss in the value of their currency, and loss in confidence the government actually knows what it's doing.