It's not just gameplay utility that makes these cards costly -- it's the difficulty of finding physical copies of those cards, hence players' constant demands for reprints in Masters sets and even Commander deck cycles
The flexibility of this ability - and the fact that you can hit each one in a single turn in the right deck - means youll be generating absurd value constantly, just by playing other creatures or making creature tokens
Traveling Chocobo's last ability affects a permanent's own "enters" triggered abilities (as long as it's a land or Bird you control) as well as other triggered abilities that trigger when a land or Bird you control enters the battlefield
Furthermore, third-party enthusiast sites like TCGplayer or even grading services such as Professional Sports Authenticator (PSA) have indexes that allow you to see, perhaps most importantly, the historical trends of raw and graded card prices
Four mana for two is also behind rate
Currently the format is everything released between OP-01 and Op-07, along with every starter deck