Why non Muslims are not allowed to marry Muslims unless they convert to Islam?

but still thousand of Muslims marry each year in almost every country people who believe in other religions. I think this is totally unfair.
I think Islam should allow non-Muslims to marry Muslims without convert to Islam. then that's the true religious freedom.
Update: God rules? how hypocrites.
Update 2: @Jonathan
My sister is a Buddhist and my brother-in law is a Christian and they have been living well for 5 years.
because both Christianity and Buddhism don't force religious rules nowadays(Buddhism doesn't have any rule ) so there is no issue at all. so don't lie. it's only an issue of Islam.
Update 3: @Lisa
Buddhism never says non-Buddhists go to hell. it's only people who do terrific works.
you may believe Islam or whatever anything but if you do any harm to anyone even by the name of the religion it's a sin in Buddhism.
Update 4: @Nisse is here
Is that why Muslim men crawl Europe to grab Christian girls? then later force them to convert Islam?
I have seen a ton of incidents like this. but no Muslim family gives their daughter to a non-Muslim man unless he comes to Islam.
Update 5: @Rozeh
I think you are the only Muslim I came across who admits what really happens out there.
most of Muslims never admit their faults instead they justify it with a lot of gibberish.
glad to know there are people like you exist.
Update 6: @Farooq Raz
Buddhists doesn't worship gods.
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