A large EITC is trivially converted into a UBI. Otherwise-unemployed people pay each other a salary to do whatever they were each going to do anyway and then the salaries cancel but they both get the EITC. Attempts to prevent this largely don't work against people who want to do the scam and just find a different way to structure it, but tend to be used as an excuse to e.g. screw legitimate self-employed workers out of the credit.

The only reason this doesn't happen at scale right now is that the existing EITC is smaller than the amount of payroll tax you'd pay by making a given amount of money, i.e. it's not a negative income tax at all. If you're going to make it larger than that you might as well make it a UBI.