Cap isn't in the same ballpark as Clark when it comes to willpower, inspiration, or in bravery.
Cap is kind of an *******, instead of talking to Cyclops he stormed an island with 2 Hellicarriers full of mutants so they can take Hope by force
He's quit being Captain America several times, the most notable example in his first volume when he couldn't get into Sharon Carters pants, he threw a fit and quit.
People talking about how Superman is powerful, when you routinely face guys like Doomsday, Zod, Bizzaro, Henshaw, Darkseid, Parasite, Mongul etc...it kind of balances out when Cap routinely faces guys like Red Skull, Crossbones, Batroc, Winter Soldier, Baron Zemo etc etc
Hell even when you look at Superman's human enemies, Lex Luthor is a bigger threat to the world than Cap's entire rogue gallery combined
Cap is to Superman what Spiderman is to Cap, Cap would probably beg Superman to be his sidekick.