[ndt]

Why are judges above the law?

They are not.

Why can't we investigate them to be sure they haven't been compromised?

They can be investigated if they have broken the law and can be impeached by congress. Judges don't exactly work in secret, their opinions are published (even if classified, they still provide detailed opinions) and they have to explain exactly why they decided the way they did. If their opinions are questionable, you appeal.

[oceanview]

sure, let us know when the Kelo SCOTUS justices are impeached. or any of the ones on the 9th circuit who routinely invent new rights and ignore others, under the guise of "interpretation".

[airborne]

If their opinions are questionable, you appeal.

And if their integrity is compromised?