a significant part of custom loops are just (or mostly) for fun.
and when people already have a custom loop, the do experiment and doing fun things.
It's a hobby. Can be an expensive one. That's why people don't bother with custom loops unless they already have high end system.
Practically, there are very few reasons to do liquid cooling. The only one I can think about is cool and quiet compact system. Otherwise, you can achieve the same with air cooling.