Didnt see posted anywhere else, but...
From Freshly pciked Vana'diel 25 March 2017:
When service for FINAL FANTASY XI started in Japan, players were able to fish up Excalibur in a certain area. At the time, we hadn't introduced anything like relic weapons, so they added Excalibur as one of the rarest weapons you can catch. The rate was something close to 1 in 20,000 to fish up this weapon. Looking at the data, we found out that no one was able to catch an Excalibur from fishing. The team in charge of fishing probably added this thinking even if players were able to get it, they wouldn't be able to equip it, since the level of the weapon was set to level 70 when the level cap was still at level 50. This surprised us when we found this out later. If a player was able to successfully catch Excalibur from fishing, we would've never seen Excalibur created as a relic weapon! Also just to note, the item ID for this weapon is different from the relic weapon version, so the stats on this weapon were much lower than the current Excalibur and were probably closer to Ridil's stats.