Because first thing financial industry would use it against the poor, get them lump sum and eliminate benefits of basic income.
I am sure this can be made in a way that wouldn't be limiting too much, yet prevent obvious abuses you are bringing.
Yes, the poor would be disadvantaged by this. The solution isn't to take away everyone's ability to take loans on BI; this would leave people unable to take loans on their BI once they've got a stable income. The answer is to educate, to explain that taking that lump sum leaves you unable to use BI later when you need it. At most, pass a regulation stating that loaners are liable if they don't make it clear that the loan is paid off with BI over the course of X years.
That being said, the point of a basic income in the first place is to introduce financial stability to all households so any costs associated with preventing lending on basic income collateral may be societally optimal.
The reason why banks require evidence of ability to repay the loan is because, without them, there would be a huge incentive for borrowers to take loans without paying them back. This is where the adverse selection comes in - the information asymmetry results from the borrowers knowing what their intent for the loan is and the incentive to mislead the lender. This leads to lenders being more wary (and implementing requirements) when issuing loans.
Anyone getting basic income should be completely free to spend it any way they want. That is essential.
Such as to pay off a loan? Therein lies the problem: if you make it illegal to lend money explicitly based on BI, and the debtor pays with BI, who is liable? And if someone is liable, is the debtor truly free to spend BI as they wish? If nobody is liable, then what stops me from claiming that my BI is instead my paycheck, and then taking a loan based on it?