Watercooling and AIOs work on the same principles, but on different scales and levels of performance. Both use coolant, pumps, blocks and radiators, but the thermal capacities of each due to pump flowrate/volume, block efficiency, radiator volume/efficiency/materials used and of course, fans to move air through the radiators in question offer differences in how both perform.
That $100 AIO comes with a pump, fans, radiator, tubing, coolant in a single box. A single, quality watercooling pump or block might cost much more than that on it's own. Similar to how a Kia and Mercedes are both cars and work in the same manner, AIOs and custom watercooling offer price and performance in a similar comparison.