Because the jewish religious law (which many jews have to follow to peer pressure, even if they are not religious by themself) explicitely forbids marrying non jews and no one can convert to the jewish religion.
Muslims on the other hand can marry non muslims, but are supposed to convert them. So this is indeed way more comon.
Citation needed / this is not correct. It isn't easy, but it is perfectly kosher.
Last time I checked - it seems I read a viewpoint from a rather orthodox rabbi (but his article was the first one, that showed up in google at that time, now I cannot find it anymore), which clearly stated, this is not possible at all, with no exception.
The only way, would be to recognized as a "lost jew", meaning being of jewish origin, who lost connection to the tribe (even some generations ago). And the recognition would need years of devotion.
It is not aligned with the vast majority of Jewish views about conversion to Judaism.
In general, conversion is possible. As far as I can tell there are even clearly-enough defined requirements.
Jews are not supposed to treat converts any differently than non-converts. People being people, this doesn't always happen, but that is the reasonable principle.
Given that Jews for centuries have not proselytized to non-Jews, many believe you cannot convert to Judaism. You can.