That’s where you are mistaken …
a) legendary runes most common place to drop … T4 bosses
b) second best source to get legendary runes - playing with runes combo at jeweler (I got that way 5+ legendary runes) but not 3 legendary for 1 rather 3 rare for 15% chance of legendary 
c) Runes drop probability not equal across the world … new zone/specifically new dungeons one of the better bets (and undercity/tribute harmony)
d) you missed 1 more or less good source of runes - Realmwalker event under opal of materials