Why is Judaism as a culture so much more obsessed with Jews by Birth than Jews by Choice? Why do people default to assuming "Jewishness" of those born to other Jews, even if they are practicing Buddhists, rather than those born to Christians, even though they practice faithfully in the style of Modern Orthodoxy or even Charedi Orthodoxy?
I think it is because of the fear of choice, and a lack of confidence in being Jewish in a variety of ways.
With regard to choice, choice is a variable. Humans don't like variables such as choice unless they are not so much a choice as a guarantee in their favor. They like to choose their own dinner. They don't like other people choosing their own dinner so much. They might choose something that makes them gassy and fart during the cocktails later. Indeterminacy, flexibility, openness to change, and a wide variety of other appellations are automatically open in the fearful and pessimistic human mind to going wrong. If it can go wrong, it will.
Jews by Choice can choose differently later on. They can join now, leave later. Of course, so can a spouse, but that doesn't stop us from getting married. We enact, if we have sense but for other reasons, strong discouragement of divorce, but it is still one of those might go wrong deals. So too are welcoming converts. They might proselytize for a heretical ideal. Ooh, scary.
The other reason is of course lack of confidence as in the response which various rabbis have remarked on which goes along the lines of "are you insane? Have you seen all six hundred and thirteen mitzvot? You'd actually choose this life on purpose?"
Well, I guess you might if you didn't see it as so much of a negative burden as they do (those who make those comments).
Jews by Birth on the other hand are born to it. You don't choose your parents, you don't choose your family's traditions, you pretty much get no choice. They chose to get it on, have kids that result from getting it on. Those who come before you got all the fun choices. You got stuck with a nearsighted dad who rants at you for bad report cards before he finally sees that it was actually a B and not an F and a mom who can't tear herself away from soap operas long enough to make cookies with you.
Also, you always have the old cudgel of the familial relationship to keep you in line. You might have the ability to choose later, but for right now, until you're an adult, you have what, eighteen years to have "being Jewish" reinforced as much as Christians have?
The choices of others and their possible decisions to reverse those choices to our detriment... I think that is what makes the birth rather than choice nature so much more concentrated on. Of course, there was the message firmly beaten in by Rome, Christianity and Islam to not proselytize, and the evolutionary move from faith in miracles and G-d towards rote ritual observance in its place, but I think that fear is the big reason for obsession with birth over choice and I think its not different when it comes to immigration.
However, to those we put through the hoops, we don't usually turn on them and suspect disloyalty just because let's say a naturalized Mexican puts a Mexican flag sticker on his car the way a fifth generation American of Swedish descent puts up a Swedish flag sticker on his car. Forget to observe faithfully, and in religion, it's not so well regarded or felt harmless. Which likely has to do with the rote ritual observance thing being what we place more importance on.