hahaha....
what I think here is... if all the monster do exist, they will stay their "human form" be a soldier or a president of some country ... make a war.... or at least make other country start a war and then they pretend to be savior by sending their soldier to "save" the country who that rule "bad" president/prime minister... when the president/prime minister has roll down,no one(human) will rule ... so they make some of their clansman pretend to be that country minister to prime minister and rule the country... no one can fight them if they stay in their "human form" because no one know... there will be no rider or sentai/ranger...
(unless the monster/kaijin are "STUPID" to wreck a havoc in their monster/kaijin form)....