It's basically be because the natural tendency of government is acquire and centralize as much power as possible. Now some forms of government take longer but without constant review and concern by citizens in a democracy it will gradually move towards a despotic regime. In some countries like China it has already happened because individual freedom isn't that valuable to the general citizenry, in the USA and Europe it is moving in that direction. Citizens need to become more active and vote for parties that advocate for more freedom rather than less.