[Ali Imran]

From my observation, those that converted to Islamic religion are all because of marriage to a Muslim spouse. It seems like Muslims will never convert to other religions for the sake of marriage.     

Yes, I'm sure you probably know at least one who converted to marry a Muslim spouse. But many of such cases ended up with a pious convert. I know a few myself, and I met one just 4 weeks ago at the mosque. He is an Englishman and married a Malay woman. 

But the number of converts who converted out of conviction is probably bigger. We see a lot of them on YouTube. They became convinced after studying. They started by learning about the faith before believing, some took years to convert finally. Many of them became lifelong students of Islam. You will find this type of conversion in almost every corner of the world, even in Japan and South Korea.