Because it's about obeying Allah. Islam is not comparable to Christianity. While the muslim believes the Quran to be the word of God (all of it) and that it was preserved since the time of the prophet Mohammad, the Christian has none of that. The Bible exists today in thousands of different versions, it is not considered to be the complete word of Jesus (except by ignorant Christians). A verse (in John I believe) that spoke of the core of the Christian belief, the trinity, clearest in all of the Bible, was taken out from the Revised Standard Version as a fabrication. You can probably see where I'm going with this: They do not have a solid foundation like we do. Therefore it only makes sense for them to not be religious.
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Allah deserves to be obeyed, and how do we obey him? By following what he says and what his prophet has said. Your parents made mistakes and they should repent. You're obviously not supposed to follow in their footsteps when it comes to the whole boyfriend/girlfriend thing as having a sexual relationship like that is a major sin.
There are many educated muslims that have produced video material online, I really do recommend you check them out. Names I know of are: Ahmad Deedat (counters Christianity), Hamza Tzortzis (counters Atheism), Abdul Raheem Green.
Not all people who supposedly preach about Islam are good though, if you have the correct understanding of the basics of Islam (for example that Allah is one, what is considered shirk, that Allah is the only one who deserves to be worshipped) you will be able to eventually differentiate between the good and the bad, that is when you have enough knowledge about Islam and about the people in question.