1. Some investors want the investment return of a mortgage, but aren't structured or licensed to originate loans. So the loans become packaged in commoditized product and sold around to these people who are pension funds and endowments and insurance companies etc. (See Mortgage Backed Securities or Collateralized Mortgage Obligation). Also different financial institutions are always looking to balance their risk profile or liquidity level, so this is just 1 type of debt asset they hold vs treasuries, junk bonds, corporate bonds, etc.
2. It's a financial product, so it gets packaged into whichever way the market demands it. Sometimes they mixed riskier debt with safer ones to reach an target risk profile. Often now they package like-debt together, because people seek it. Like people want a basket of mortgages from a certain geography because they have a mandate for that exposure, or that location has lower interest rate sensitivity due to local laws or whatever.
Can I get a beer too?