By your standards, MBS are also not particularly complex. They're not that different from pools of mortgages. In fact they have a lot of similarities with index

By your standards, MBS are also not particularly complex. They're not that different from pools of mortgages. In fact they have a lot of similarities with index funds and tranches are similar to options on the index. The complexity is precisely in the details. How much is the pre-payment option on your fixed rate mortgage worth? How much is the embedded interest rate swap that differentiates a fixed rate mortgage from a floating rate mortgage worth?