Generally speaking, because those were not their actual names. In both Chinese and Japanese (as well as other Asian) monarchies, the Emperor was generally not referred to by their given name, but rather by what they named the era of their rule. Especially posthumously.

For example, Qianlong was the name that the emperor (who's real name was Aisin Gioro Hongli) chose as the name of his era when he took the throne. (Edit: u/EnclavedMicrostate corrects me that Hongli or Hung Li are more accurate usages.) When you translate into English, saying "the Qianlong Emperor" makes clear that you're talking about "the Emperor who ruled during the Qianlong era." If you say "Emperor Qianlong," then English speakers tend to assume Qianlong was their actual name, similar to how we say "King James."

In a lot of Asian nations, before the adoption of the Gregorian calendar, the current year was generally referred to by the year of the current emperor's reign. In many government documents, this is still the case. For example, the current year in Japan for government purposes is "Reiwa 2," because it's in the second year after the current Emperor (real name Naruhito, era name Reiwa) took the throne.

It should be noted that in both cases, people generally do/did not refer to the current Emperor by either name. They are just "The Emperor." It's only once they pass away that they are referred to by era name posthumously. In some cases, if the emperor was semi-mythical or just too far in the past, we don't actually have records of what their real name was, only era name.