Tourist visas from these countries are usually denied for single people with no family connections in the U.S. if the consulate knows you have a relationship with a potential spouse, or even a boyfriend, it will always be denied. The reason is as stated by MsMagoo that people just overstay their visa. Very generally, If they end up marrying a U.S. citizen that overstay is waived and not held against them. it isn't easy to get into the U.S. from many countries. That is part of the reason why people use the fiance visa. In addition to the poor financial situation of everyone on the show which might prevent a visit, the U.S. won't allow many people in for a visit.
I'm honestly shocked that Danielle was able to get Mohammed a fiance visa with her precarious financial situation and her past frauds.
Mohammed is trapped, he can't leave and ever hope he will get another fiance visa granted, because it looks like fraud. This is his only chance. What a disaster that turned out to be.
But I doubt any of these relationships last much more than the minimum necessary, and maybe not even that long.