Loans need to be paid back. They have to either have income or sell assets to have the money to pay the loan back, presumably with interest. Both of those are taxable events. Stock-collateralized loans don't net tax-free money.

What we really should do is crack down on NGOs and "charitable foundations" that are being used as tax shelters, almost exclusively by the wealthy while either doing nothing or actively destroying society. But the left would never agree to taxing that wealth because it's how they exercise power.