Like other financing transactions, securitization affects a firm's capital structure. But clearly, firms spend enormous time, energy and expense choosing among, and sometimes developing, different capital structures. This behavior can be seen as rational only in a world where the theorem's underlying assumptions are violated, and the theorem therefore does not hold.
Financial markets are perfect.
This also means that there are no transaction costs to issue, buy, or sell. Financial markets are complete. For my purposes, this means principally that perfect substitutes for everything are available. The first assumption, coupled with the traditional finance principle that a firm's value is the net present value of its expected future cash flows, implies that there is only one correct value for a firm, regardless of its capital structure. And it is not only differences between equally valuable firms or cash flow streams that will be arbitraged away.