I'm a non-libertarian Republican, and I think that the two ideologies have a lot in common, but that does not mean that a Conservative might as well be a Libertarian and vice versa. I support an interventionalist foreign policy when necessary, believe that certain programs are necessary for national security, and think that the government does have a role in enforcing a certain moral code (i.e. not being able to kill unborn children), which puts me at odds with the Libertarian wing of our party.
Republicans who are on the wrong side of things never fail to amuse me with such gems as trying to make abortion out to be murder.
Anyway, it's because Libertarianism is something you grow out of once you're older than 18.
Now, to your second part, which I'm actually more eager to rebuke.
How come libertarianism is considered a childish philosophy? It's in fact the simplest and, if I do say so myself, most logical mainstream political philosophy there is. Limited government. That's it. We mostly agree with liberals on social issues, often taking it further, and we mostly agree with conservatives on economic issues, again often taking it further in favor of limiting the role of government. We also have a non-interventionist foreign policy--again, this ties in to limiting the role of government.
It's a simple ideology, and just as valid as liberalism and conservatism. Is it more theoretical than practical? Yes, because we've never been given a chance to run the country due to the inefficient voting system. But it's still valid, and it makes as much sense as others. It's not contradictory, and it's well-defined along all issues.
To us, liberty is a more important value than enforced "equality" or enforced "morality," and the best way to achieve liberty is through reducing the influence of the government. I'm baffled that people think this is such a crazy thought.
Libertarianism has that reputation because of it's simplicity. Stripping the government down to a constitutionalist shell and trusting the corporations of the "free market" to regulate themselves and manage the economy in a benevolent manner is naive.
