The following is a chapter of Islam 101:
This question is a bit like asking, "If Christianity teaches humility, tolerance, and forgiveness, why are so many Christians arrogant, intolerant, and vindictive?" The answer in both cases is obvious: in any religion or ideology there will be many who profess, but do not practice, its tenets.
Just as it is often easier for a Christian to hit back, play holier-than-thou, or disdain others, so it is often easier for a Muslim to stay at home rather than embark on jihad. Hypocrites are everywhere.
Furthermore, there are also people who do not really understand their own faith and so act outside of its prescribed boundaries.
In Islam, there are likely many Muslims who do not really understand their religion thanks to the importance of reciting the Quran in Arabic but not having to understand it. It is the words and sounds of the Quran that attract Allah's merciful attention rather than Quranic knowledge on the part of the supplicant.
Especially in the West, Muslims here are more likely to be attracted by Western ways (which explains why they are here) and less likely to act violently against the society to which they may have fled from an Islamic tyranny abroad.
However, in any given social context, as Islam takes greater root — increasing numbers of followers, the construction of more mosques and "cultural centers," etc. — the greater the likelihood that some number of its adherents will take its violent precepts seriously. This is the problem that the West faces today.
Islam 101 was written by Gregory M. Davis, author of Religion of Peace?: Islam's War Against the World, and the producer/director of Islam: What the West Needs to Know.