> Two Korean presidents were sentenced to death and were pardoned in the 90s.

The important context is that these two presidents were Chun Doo-hwan and his successor Roh Tae-Woo, who led the military coup of December 12th (1979), seizing power, and then sending paratroopers to murder hundreds of civilians to quash public protest in the uprising of Gwangju (1980).

They weren't your garden variety corrupt politicians. They were mass murderers, and by 1995 when they were arrested, they and their military cabals were still posing a credible threat to Korea's democracy. Their arrest and subsequent death sentences, accompanied with a sweeping purge of their military cabal by president Kim Young-Sam, marked an important inflection point in Korea's decades-long struggle toward democracy: before that the threat of a military coup was a constant factor in politics. After that the threat was gone, and since then, the Korean military never even pretended they had any political ambitions.

So mock their later pardons if you want to, but you can't deny it marked an important and necessary step in Korea's history. It also shows sending your ex-presidents to prison only to pardon them later is still better than not bothering with it at all.

* Also, the "obvious reason" that American politics sent zero ex-presidents to prison is that Biden chickened out. So, there's that.