Look, I haven't dug into this, but if one wants a fair comparison, then you need to account for the size of an economy. If 330mn people need pensions, then you'll obviously see much larger pension funds. If 400mn people across 27 countries want pensions, these will often end up being national level and will look individually much smaller than the ones from the US, purely because the US has more people.
(Anyway to put another argument: the US can outflow. Why should people invest to a Trumpland?)
Many Europeans prefer bank deposits to investment in markets, that's true. I assure you though, there are lots and lots of pension funds in Europe, as well as many, many insurance companies who represent similar capital profiles.