The first reason I think is because most of the time it's free...free as in the money comes from Govt reserve and not from their pocket.They too can afford  foreign treatment as our netas are not poor and have already looted a fortune from the country,Another reason can be for some fancy  publicity--they want to show all the citizens that they are vital person to be saved for the country, which in most cases is a joke.The current state of the country itself reveals  how poor health our netas possess as no one seems to have clear vision and a good mind for cooperation and both of those things stems from a healthy body and mind.I think Nepali doctors are fully capable of diagnosing the ailments of anybody of anykind --be it neta or a citiznery--except some.like neurosurgery,which require high tech help and flawless hands,that only few doctors possessed in this world including some Nepali doctors.And beside, they cannot rightfully also say to otheres that they want a vacation when they have put the country in such a poor condition by their own deeds, so lying it as a treatment is a nice way to escape from all eyes and enjoy exclusive vacation that comes as a profit being a neta of a country called Nepal.