Why do bulbophyllum like so much water?

[estación seca]

Teleology is the human habit of explaining things by figuring out how those things might function, in a way that makes sense to a human. It doesn't always yield useful explanations.

Not all Bulbophyllum species tolerate dry seasons. Presumably extant Bulbophyllum species evolved from ancestors having pseudobulbs. Some ancestral orchids may have evolved pseudobulbs to survive droughts. Extant Bulbos may have lost the ability to survive drought, since they never experience it, but retain pseudobulbs they inherited from their ancestors.

[Fairorchids]

I want to second Ray's comments.

As a general rule, I do not allow any orchids to dry out during their growing season, and they reward me with ever increasing growths. For mature Cattleyas, that means barely reaching 'near dry' stage.

I do observe dry/rest seasons for those orchids that demand it (such as Australian Dendrobiums, which get little or no water from Oct 15th onwards).

I do carefully select potting material and pot size/type, so that I can water everything in the greenhouse on the same schedule (twice a week in summer, once a week in winter).

[Jakob]

Most, if not all, epiphytic plants have adaptations for water retention (bulbs, pseudobulbs, fat stems, hick cuticles, hairs, leathery leaves ipd.) They do not have any contact with ground water - even if it rains every day, they are going to dry out between rains. Tropical conditions are more severe regarding evaporation - during the daytime in the canopy it can get very hot and dry (closer you are to the groud, moister and cooler), so an epiphyte can easily wilt in a matter of hours without necessary adaptations.
Pseudobulbs are one such adaptation even for the plants that get wet every day, without them they'd loose too much water between rains.
Orchids that do not have pseudobulbs often have leaves that retain water exceptionally well, succulent leaves or they live in cloudforests, where evaporation is not (such) a problem.
A big and costly structure would be very quickly lost evolutionary if it had no benefit to the fitness of the plant, plants especially are very plastic in this regard, they can evolve and de-evolve structures very fast (relatively speaking). If they had no 'use' , the pseudobulbs would be gone or at least transformed into a much smaller organ.

Bulbophyllums aren't really the only ones that like to be kept moist and humid though,to my knowledge Oncidiums and most of the alliance like it too if I'm not wrong, and most of them have p-bulbs

[Bolero]

Agree with all of the above. It's about contact time between the wiry roots and the water........ it takes more effort for the roots to take in enough water due to their size. It's worth the effort though.