Well honestly, this is not a question with an simple answer. The stuff you'll be reading from this comment would be mostly stuff that aren't confirmed completely. Depending on the individual, they might say different stuff about this topic in Korea. However, I'll try to explain this in a way that most of the general Korean public would agree upon.

Also, this really is definitely a modern history question, but honestly it's more of a current society one.

First of all, Koreans have a word for this, called Sa-ibi(사이비,似而非). Sa-ibi literally means "Looks very similar from the outside, but very different from the core/inside. It originated from the texts of Confucius, and these days it's used as a word to point to religions like the ones above. It does generally have a leader who calls themselves "the Messiah/the Second Comming" or something else. However, religion that aren't in line with the general public are commonly classified as this. Basically, psudoreligions.

The number of Sa-ibis are almost impossible to count. People in these religions do not consider their religion as a Sai-ibi. Besides, most of them are hidden and the examples above are just the top successful ones. Some estimate 300+ and others say thousands. Most of them are known to be associated with Christianity, but again, we don't know exactly. Still, most Sa-ibis in the media (including dramas and movies) are portrayed as stuff related to Christianity. 

Additionally, these religions aren't that really seen as good from the general society. This news article from Yeonhap(a large news company over here) reports that 87% of Korean companies had been affected negatively by Sa-ibis through negative press and exposing employee's private information. 

The constitution of the Republic of Korea, Chapter II Article 20 states "All citizens have the right to enjoy freedom of religion". So there aren't that many ways to halt their negative effects (unless they literally do serious crime).

Then why are these Christian-based Sa-ibis popular? First of all, Christianity is extremely popular over here. South Korea has a bunch of churches. This photo, for example, was spread across the internet a while ago. Those red crosses are all churches, and this scene is very common in South Korean. Also, Yeoido Full Gospel Church in Seoul is the largest Pentecostal church in the world. Unlike other Asian nations, South Koreans rely a lot on Christianity for their beliefs. Christianity in Korea originated from the 17th century, during the Joseon Dynasty period. Korea is actually a rare case because they started to accept it as a religion without a missionary from the west. Joseon scholars got the Bible from Qing, and at the begining it was called Seohak(서학,西學), literally meaning "Western Studies". Since Joseon is also known as the hermit kingdom, Joseon haven't experieced foreign powers yet. Scholars of the Jung-in(중인,中人) class, which is a group of people who did stuff like translating and medical stuff (basically, important stuff that aren't Confucius studies), started to read the Bible as a way to learn about the west, but after reading it they started to agree with the stuff on the Bible. The Jung-in class is educated enough to read text and do advanced stuff, but not that really devoted into cultural standards compared to higher classes. These dudes literally accepted Christianity by themselves and they were the first to spread it. Due to that, there were no "weird foreigner randomly appearing" moments in the begining that may have negatively affected the spread of the religion.

Some people just simply consider Christian Sa-ibis as just another division of Christianity, even though the leader may call themselves Messiah. Like how some churches in the western world convince their followers to pour in a bunch of money to the church, those Korean leaders just may have taken one more step than others.

And note that the South Korean society is quite intense. Young people are forced to compete in a extremely strict education and going to university is "am obvious thing to do". Millenials are facing the one of the worst unemployment crisis. Various cultural aspects effect individuals in extreme social and peer pressure. People need somewhere to rely themselves on. People want to feel like they're important to others. People want to feel cared and loved. Religion, in theory, can be a place of relief for individuals. Sa-ibis can easily "abuse" this dark aspect of society and justify themselves as legit religions and "the legit god". 

Sa-ibi people can easily be found in big streets, trying to look as an ordinary person passing by. In one common situation, they pretend as if they're college students doing surveys. They dig up the person's info and slowly approach them friendly. Step by step, people are being continuously tucked into Sa-ibis in a very deep level. If you literally convince an individual to a deep moral level, it wouldn't be that hard to convince them that "I am the legit god".

TL;DR: South Korea is a society that already has a huge christian influence. Due to various cultural reasons, the South Korean society became vulnerable for psudoreligions, and those leaders just used that opportunity extremely successfully.

PM me if you need any more details, or any other question about Korean history or society. I'm open to all questions.