A passive filter is designed to work with specific terminations. The termination resistance is every bit as important at controlling the transfer function as the components within the filter. For instance an LC filter may require a 50\$\Omega\$ load at each end. An RC filter might require to be driven from a short circuit, and be loaded with an open circuit.
If you cascade a second passive section directly, you change the loading the first section drives into, and the second is driven from, changing the transfer function that each section produces.
If you place a buffer between the sections so that each section still sees the correct terminal impedance, so a 50 ohm isolation amplifier for LC filters, or a high input impedance buffer for a RC filters, then the individual sections retain their original transfer functions, and the resultant is the product of the original functions.
The transfer function of an active filter need not depend on the terminations. A Sallen Key filter has a zero impedance output, and is designed to be driven from a zero impedance input. Its transfer function does depend on being driven from a zero impedance, if that changes the transfer function will change.
However, when we cascade these filters directly, the zero impedance output of the first stage correctly drives the second stage input. Their transfer functions are automatically the product of the individual sections.