regarding CLUSTER and HUGE work_mem / maintenance_work_mem

Let's say I have a 7GB table with 3-4 indices for a total of 10-12GB.
Furthermore, let's say I have a machine with sufficient memory for me
to set the work_mem and maintenance_work_mem to 20GB (just for this
session).
When I issue a CLUSTER using one of the indices, I see PostgreSQL (by
way of strace) performing an index scan which amounts to large
quantities of random I/O.
In my case, that means it takes a very, very long time. PostgreSQL is
largely at defaults, except for a 2GB shared_buffers and a few
unrelated changes. The system itself has 32GB of physical RAM and has
plenty free.
Why didn't PostgreSQL just read the table into memory (and the
interesting index) as a sequential scan, sort, and then write it out?
It seems like there would be more than enough memory for that. The
sequential I/O rate on this machine is 50-100x the random I/O rate.

I'm using 8.4.10 (with the 'inet' de-toasting patch) on Scientific Linux 6.1.

--
Jon