Ever watch two kittens get themselves all tangled up while playing with a ball of yarn? Imagine being one of those kittens and getting tangled in string in front of an audience of other CS PhD students and senior CS researchers. In addition to the issue that we really couldn't simulate what GC was doing (or even explain it), imagine trying to untangle yourself while doing it.

To answer the question about why GCs are an issue, a GC scans memory, moving objects, updating pointers, writing marks in special headers, etc.