The cell design they had set up for the Hulk, which instead ended up in use on Loki and then Thor, is probably still they best they've got. And while it did a reasonably good job on the Asgardians, it likely wouldn't be effective on Clark unless, ironically, he consented to be held there.

There's just only so fast you can go in an interdimensional, interplanetary arms race. Humans are a little bit outflanked. They're really quite fortunate he's friendly.

And perhaps they're also fortunate that Steve is as friendly as he is. It seems wise to give Superman the best possible impression of the people he's landed amongst. Kindness is never a waste, and it can be strategic, but Steve is sometimes a simple man. He sees a strange sort of kinship here, and maybe a little responsibility.

So, it's a couple hours later that they arrive at Steve's apartment building. He goes up the hall slowly, glancing at one of his neighbors' apartments in case she's around (she's a nurse--cute, and nice; he's not smitten, but he's interested). No such luck, but he's not too disappointed, and he lets Clark in with a sigh. The furnishings are mostly vintage, with a few modern twists, and it's very tidy, except for the paintings and art supplies scattered around the living room.

"So, I wouldn't say I'm a great cook, but I can pull together some hamburgers. Or order a pizza. You hungry?" Wait, does he even eat human food? Nah, he must eat human food if he's lived on Earth for a lifetime...