I think there should be a distinction made here. Bacteria is a big word, a HUGE word. There is bacteria everywhere, on nearly every surface of everything. In aquaria, there is beneficial bacteria literally clinging to every surface in the tank. Beneficial bacteria is another big word, so lets narrow it down even more. Nitrifying bacteria, the kind responsible for the ammonia => nitrite => nitrate portion of the nitrogen cycle, is aerobic, so by it's own nature it is present in the greatest numbers in an area where there is both a high level of o2 and constant food source.
This is why the biowheel is/was so popular, it's hard to beat that amount of oxygen exposure, and they figured out that they could build massive bacteria colonies on a biowheel surface.
In most cases, massive amounts of biomedia surface are not necessary, and as mentioned, a tank without much biomedia or power filtration will find other areas to build the biofilter.
Does that mean substrate is pointless? Not at all, it serves a purpose as well, and it can also serve to some degree of nitrification and moreso denitrification, so it is 'bacteria laden' as well.