"because none of the banks sell what they hold, the price goes up; because the price goes up, more people make money on their positions; because they make more

"because none of the banks sell what they hold, the price goes up; because the price goes up, more people make money on their positions; because they make more money on their positions they buy more stuff and don’t sell what they hold; and on and on forever" That is until the bubble pops, those who got in late and can't sell will make a loss. The bid-ask spread widens, in particular there is glut of people wanting to sell at high prices, whilst there are few cautious buyers willing to offer lower prices.