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.
It's OK, but you still run into the issue of not having anything else going on for you. I think he deserved D tier because the things he can do are not things that are considered useful in the general gameplay of Warframe or are done better by other frames. Inaros is perfectly fine frame for 90% of content but he still deserved D just cause there are other frames that do what he does far better.
Yeah. Grendel is Inaros but better in all content where Health tanking matters. And he can switch to shield gating once it doesn't. Plus his abilities give him easy 100% armor strip and scaling damage. The only time where Inaros is objectively better than Grendel is if the mission prevents ability use. Any other time, Grendel is just better than Inaros is.