Krishna places what just happened into perspective. This form of Mine that you saw — it is not merely rare. It is sudurdarsham, exceedingly difficult to behold. The gods themselves — Indra, the Adityas, the Vasus — they long for even a glimpse of it. And they long constantly, without end.
Consider what this means. Beings who live in celestial realms, who have accumulated vast merit over countless lifetimes, who dwell in light and splendor — even they have not seen what Arjuna just saw. The vision was not available to the powerful or the privileged. It was available to the devoted.
Krishna is not boasting. He is explaining. Arjuna might wonder: was that real? Did I really see the form of the infinite? Krishna answers by establishing the rarity of the experience. You did not hallucinate. You did not dream. You saw something the gods themselves have been waiting for.