I believe it's because ack and friends skip files you ordinarily don't want by default. So there's no .svn-base duplicates, no cache files generated by tools, etc.
If your find command looks like
find . -name '*.pl' -o -name '*.pm' | xargs grep foo
ack foo --perl
But I just timed the time it takes to type those, and they took me 9.2 vs. 2.3 seconds.
So which is faster: 9.2+1.0 or 2.3+1.5?