Because with cash you have the money to actually make the deal happen. With a mortgage you depend on a bank giving you the money. A bank says they will lend 75

Because with cash you have the money to actually make the deal happen. With a mortgage you depend on a bank giving you the money. A bank says they will lend 75% LTV, you are preapproved for a $2M house, so will bring $500K to the table and bank will provide $1.5M you think. But wait, the appraisal comes back for $1.6M instead of the $2M you bid. Bank says no problem, we will lend 75% LTV or $1.2M. Now you have to find (within days) $300K. And you can't borrow it because that will mess up underwriting on your current loan. Or you buy a car before closing. Whamo - your DTI is toast. Or you have someone open a CC in your name. Whamo - no closing.