For the moment I still don't understand.
[~snip~]
And I do a brute force, and the brute script tells me that the words : word1 word2 word3 are a valid wallet, it's over ?
What prevents a person from having access to the wallet?
Your wallet private key is a number within a hugely big interval. Let's say bigger than the number of stars and planets in the universe.
And yes, if somebody finds out that number, you're screwed.
Just the chance is smaller than winning the big prize at lottery many times in a row (!).
The seed is indeed "just words". But keep in mind that being a rather big set of words in the list your seed words are taken from, and even more, a word can be used multiple times in your seed, makes it, again very (very-very-very) difficult to brute force it. So difficult that the electricity needed for the job doesn't worth it, no matter how many coins are in your wallet (!), plus it takes an awfully lot of time (many generations!). Again, the chances to "guess" or brute force it are extremely low.
Clearer?