Not a historian, but some possibilities:

- some governments were explicitly modeled on the US system

- others were influenced by the US system as they moved from e.g. monarchies

- most countries have some sort of caste system that established interests want to preserve

Non-proportionately? For example the Netherlands has a senate but the weight of senators per province is set by population. They don't let Saba have equal powers with Utrecht, which is exactly what the American system does. Other Anglosphere countries — all of which have exceptionally bad forms of government due to the legacy of England and the early influence of the United States Constitution — have upper houses that do not have America's weird geographic correspondece.

There is a particular sort of partisan who loathes any process, procedure, or rule that acts as an impediment to his agenda. Never mind that, quite often, these same processes, procedures, and rules often act as impediments to his opponents when they are (temporarily) in the majority, he sees his faction as ascendant forever because the universe is designed to promote his peculiar idea of progress and thus there is no longer any need for those hurdles and obstacles. In hushed whispers he might even confess he thinks there never was a need, that those were put in place by his enemies to thwart his righteous cause.