The program would be simple. The hardest part is getting a reliable source of random numbers. Once the program is written properly, it still must be adapted and tested for each popular computer platform. RFC 1750 points out that good random number generating techniques are not necessarily portable. The source code for each version must undergo public review and the object programs have to be distributed in a trustworthy way.
Users then have to download their copy, unpack it and learn how to use it.
Most users will not bother to authenticate their copy even though it could easily have been be doctored to produce predictable passphrases. In fact, it is so easy to concoct a program that appears to be random but only produces passphrases from a restricted set, that I would be reluctant to use any passphrase generating program I did not write myself. One exception might be a passphrase generator built into PGP itself, since I trust the process by which PGP is produced and distributed. Finally any password generating program is subject to a whole range of electronic attacks even after a verified copy has been installed on a user's machine. In particular, the passphrase generator has to display its output, making it subject to Tempest intercept.