What generates the microseism?
The microseism is ocean waves that couple to the ground to make Rayleigh waves.  The ocean waves are gravity waves which are dispersive (different frequencies travel at different speeds), and 0.2 Hz (not 20 mHz) is the frequency of the waves that travel at the same speed as storm winds to which they couple.

The ground motion doesn't die off, even thousands of kilometers inland, because Rayleigh waves are bound to the surface (so waves from a point source would spread circularly instead of spherically).  The waves are produced all along the coast, which negates the circular spreading.