[particlewave]

Pore diameter in the filter media is restricted by the need for fluid flow. In other words, there are many ferrous metal particles that are small enough to fit through the pores of the filter media and if those pores are made smaller, the oil will not flow freely enough and/or the filter will become clogged by debris prematurely and cause either complete filter bypass or oil starvation.

Magnets in oil filtration systems are actually pretty common, just not on passenger cars.