People won't do this for the same reason they will not sell a home for physical cash: taxes. This is a problem for both the buyer and the seller. Many real estate lawyers will not close on a property if physical cash is involved, and will ask cash be deposited in a bank. The same scenario would play out in a bitcoin sale.

For your second question, when people have a low interest rate mortgage, and the government keeps inflating the currency supply, you are effectively paying off a mortgage with cheaper and cheaper dollars. This is why mortgage interest is front loaded. The banks make dollars up front, then flip the note on the open market, and so it goes until government entities buy up all the mortgage notes. Because these 30 year fixes rates are government sponsored, and the banks know they will continue to print currency, they want to make those dollars early, before the next round of inflationary currency printing.

Very few countries have 30 year fixed rate mortgages. That is a privilege of a global reserve currency, a printing machine, and government sponsored housing.