The technical reason is probably “that is how the system is designed.” The underlying reason, I suspect, is because tournaments tend to take a very long time. and the more players, the longer it takes. See The neverending story uhuhu uhuhu uhuhu for some nightmare stories.
There is no master list accessible to non-moderators. Besides, there are over a million accounts, and a significant proportion of them are alt accounts or dead accounts (banned or abandoned).