Well,

For a significant part of history, they did not rule until death—as a matter of fact, for all of documented history, abdication was extremely common (if you'd look a bit more closely, you'd notice that about half of the reigning emperors simply abdicate in favor of someone else).

Especially from 986 to ca. 1433, the primary mode of government was rule by the abdicated (cloistered) emperor: an emperor would abdicate as fast as possible in favor of his designated heir and just keep ruling. Emperor Shirakawa (1053–1129, r. 1073–1087) is remarkable for having placed not only his son, but also his grandson and great-grandsons on the throne, all the while remaining the most powerful man at court. And yes, these acts did include forcing the reigning emperor to cede the throne. Abdications became more uncommon in the 15th century, with the practice of cloistered rule by an abdicated emperor dying out as a consequence of a major shift in the political landscape (warrior rule became the dominant factor), but still about half of the emperors until 1868 abdicated.

The reason for an abdication (esp. outside of the aforementioned timeframe) would need to be examined case-by-case, but is very often politically motivated (some cases of illness are to be found out there, as well). In some cases, emperors may even be enthroned as an interim solution in order to smooth over factional disputes at court—a political compromise on time.

Godaigo (r. 1318–1339; although his case is a bit complicated), who I will mention below again, was such a case: he was only a compromise reached between the supporters of two other (and preferred) candidates until they could sort out their differences, but then he simply refused to cede the position as intended, garnering the support of the Kamakura shogunate (this eventually led to the fall of the shogunate, but that is another story). Empress Gosakuramachi (r. 1762–1771), the last female Tenno, was another interim solution, who took over after the untimely demise of her brother Emperor Momozono (r. 1747–1762), before ceding the throne to her nephew, Momozono's son.

This heavy reliance on the mechanic of abdication is essential to understand the, on average, shorter reigns.

​

Either way, there is another factor which is quite different when compared to the average European monarchy: Japanese emperors practiced polygamy (until 1868). And suffice it to say, there is no need to be sitting on the throne to give in to the pleasures of life!

For example, Emperor Godaigo fathered 36 children—and there were a handful of emperors who had a few dozen more than this! (IIRC, the record is somewhere over sixty, or even eighty? My memory is a bit fuzzy on this.) That’s just a few bit more potential heirs than any European dynasty could offer :)

On principle, all imperial offspring, as members of the royal family, were eligible to become Tenno—it didn’t matter who the mother was, the idea of a “bastard” or "unlawful child" didn’t exist here. In theory, anyone part of the imperial household register (i.e., “royalty”) might probably be eligible (it would require rather extensive research to dig out the situations where the imperial succession was problematic and they had to take extreme measures, and why). Either way, in practice, no one looked past the third generation: in other words, the reigning Tenno, his children and grandchildren, his brother(s) and their children. I should note that imperial “princesses” were destined to remain unmarried (which also means that in the few cases they indeed did ascend to the throne, they were almost inevitably considered a temporary “solution”). Of course, there almost never was the need to look past the first couple obvious candidates—but if something horrible would have happened, they had quite a large pool of potential “backup.”

This only changed with the inception of the modern Meiji state and the promulgation of the Imperial Household Law (still in effect today), which barred most potential candidates—especially women—from the line of succession. And this, coupled with the shift towards monogamy (because that’s the modern and civilized way) rapidly decreased the number of potential candidates for the throne in our times.