The mortgage industry works a little differently in the US than it does in many other parts of the world. Mortgage loans are treated as commercial paper, which means that lenders can convey and assign them freely. That results in a situation where financial institutions bundle mortgage loans into securities that people can invest in.
he purpose of this system is to quickly free up money for the financial institutions to lend out in the form of new mortgages.
The US also has a number of government-sponsored enterprises, such as Freddie Mac and Fannie Mae, that exist to facilitate this system. Most mortgages have fixed rates, which is also a departure from the variable rates that are commonly found in Europe and elsewhere.