Why do the Egis appearance look awful and unimpressive

[Nalien]

True, it's just from a FF series perspective, it makes more sense for the Summoner job to summon them then just random people.

But it doesn't from a MMO perspective. Summoner is one of the things you think of when you think Final Fantasy and it's really one of the most well know, overpowered, and flashy Jobs in the game. FFXs story revolved around the Job. In a MMO though, all Jobs should be equal. Again, Summoner is already overpowered in these regards even despite the work they put in. Black Mage gets Flare. White Mage gets Holy. Dragoon gets Dragonfire Dive. Summoner? That gets Aerial Blast, Earthen Fury, Inferno, and that's just the "ultimate" moves, as well as a gap which will only become greater when Summoner gets Leviathan, Ramuh, and Shiva -Egi.

I'm more concerned as to why we didn't get Leviathan on FFX though

Wasn't Sin basically Leviathan? I seem to remember it basically being a summon Yu Yevon used to protect himself while he turned himself into a purple balloon. I felt it was a throwback to FFIV; Sin was the ultimate summon in Spira, in IV Leviathan was the King of Summons.