Because that's all he can really do. Also, he is hardly invincible, unless we're calling all frames with shields invincible now. There are a ton of frames that can be invisible while also having some actual useful abilities outside of that. Ivara, Ash, Octavia, hells even Wisp are all much better frames with invisibility. Switch Teleport gimmicks are the only thing Loki has going on for him, and that is nowhere near enough to make him good.
IMO Yareli is S-tier. She's an absolute tank if built right and can solo SP, Archon Hunts, and Netracells without much effort. I guess the biggest challenge is mastering k-drive controls and getting a decent secondary to use with her.
I... actually agree with most of these! I'm surprised! There are some frames that I would definitely not place as high as they are (Limbo, Vauban, Lavos, Banshee, Nekros, Voruna) but the vast majority are pretty spot on IMO.
Edit: I somehow overlooked Chroma, that boy deserved the D tier for certain, don't know how he snuck into C. Also, Kullervo should arguably be S tier but he is rather new and people might not know how insanely cracked he is so it's forgivable.
Chroma got C because if you stick nourish and combat discipline on him you can very easily maintain all his buffs permamently while also buffing teammates and looking like a guy in a happy dragon fursuit. He essentially becomes a very tanky weapons platform, like Inaros (pre rework) but with a big weapon damage buff.
That just sounds like a worse Revenant/Rhino/Nezha. Chroma's damage buff is also way worse than any other high power strength frame with Roar subsumed because it's additive, not multiplicative modifier to weapon damage sources (not to mention how Roar double dips on all DoTs, which are the meta damage types). I still firmly believe D tier is appropriate for him.
Considering Volt is genuinely meta for pretty much every activity in the game (Eidolons, Profit Taker, E/SO, general mission/relic speed runs, Aya bounty farms, level cap runs in Disruption and Cascade, etc), I'm surprised he didn't get S tier