Huge swaths of the economy depend on borrowing money cheaply to invest the borrowed money in things that will yield a higher rate of return. Not just in consumer spending but in business as well.

Blackrock doesn't necessarily want to spend a billion dollars on houses because that would mean they had a billion dollars worth of something that doesn't earn a return (cash) instead of a billion dollars worth of something that does (houses). They'd rather go to a bank and say "we have ten billion dollars worth of houses all earning 5%; give us a billion dollars at 1% so we can buy another billion dollars' worth of houses also earning 5%"

If interest rates go to 6% that doesn't work anymore and would cause major problems when the loan term ends and they have to roll over the principle into a new loan. Because the principle on the loans never gets paid back for the same reason they were taken in the first place: the money is worth more re-invested in the business than being used to pay down the loan.