This structure only applies if the story begins with the hero already a hero. This is not true of every story.
You're right that the protagonist starts out weak in probably the two most noteworthy examples of adventure fiction - Lord of the Rings and Star Wars. However in your first post you said, "The hero almost always loses at first". I think 'almost always' is going too far, in fact I think that in most adventure fiction the protagonist is shown to be competent at first, that it more closely follows the 3-act structure. One could say that the distinction here is between high fantasy - epic story, protagonist starts as a neophyte, follows the hero's journey - versus pulp - protagonist is a tough guy hero who gets the girl and punches the bad guy in the jaw over the course of 30 pages with a lurid cover.
Generally in adventure fiction, the protagonist starts out strong. Examples:
Hercules - strangles two serpents in his cradle
Lancelot in Morte d'Arthur - the first mention of him, in Book VI, says that he surpasses all other knights
Robin Hood
Batman - an adult in his first appearance, his origin story was published in a later issue
Sam Spade in the Maltese Falcon
Numerous heroes of the pulp era such as The Shadow and Flash Gordon
Looking at the most well-known fiction that inspired D&D, it's a mixture - in some of it, particularly the sword & sorcery, the hero starts out capable - Conan, Fafhrd & the Gray Mouser. Cugel the Clever isn't a stand-up fighter like Conan, but he's good at what he does - he sneaks about and dispatches foes, though his schemes usually fall apart. Elric breaks all the rules, he's simultaneously both incredible powerful - a mighty sorceror and swordsman - and incredibly weak - dependent on drugs, Stormbringer and summoned entities that never do as he desires. Lovecraft's heroes are weak, I would say, certainly compared to what they face. They're not pulp era tough guys. Tolkien's main protagonists are also weak, though there are lots of combat capable characters to back them up.
It's not true of adventure fiction in general, or of the subset that inspired D&D, that "the hero almost always loses at first".
However I should add that D&D has a very distinctive feature - the level track. There is a huge gulf in power between 1st level and high level characters. This is seldom seen in other rpgs and almost never in fiction. Luke in Star Wars and Superman from '38 to the mid-40s (though he's not really analogous to 1st level when he first appears) are the only examples I can think of. Because of this aspect, and others, D&D doesn't really model fiction at all. As has been said, it's its own genre. This creates a big problem when trying to find support for a game feature, such as losing at the start, in fiction.
It's a bit hard to say just how weak 1st level PCs are in D&D. It's relative to the world around them and that can vary greatly. For example in Forgotten Realms, where the typical barman seems to be 10th level, 1st level is weaker than a child.
