I think the answer to your question actually lies in something you wrote, let me place the emphasis:
isn't the brain adapted so the physiognomy, tongue and vocal folds work as little as possible to pronounce clear distinguishable words in those languages?
The issue is that some languages are structured in such a way that makes it hard to distinguish words unless you can clearly hear all vowels.
Japanese is a great example of this phenomenon. Japanese does not have a schwa sound, it just has five vowels, "a,e,i,o,u". It has some reduction or shortcuts that happen in typical use, but there is no vowel reduction like in English. The reductions or shortcuts typical are of another nature, like leaving out syllables in certain words, or omitting certain vowels in certain syllables in a predictable pattern (i.e. "su" becomes just "s").
In Japanese, nearly all possible syllable combinations made of commonly-used sounds, produce real words. Furthermore, there is considerable overlap: some words are distinguished only by pitch accent, and some words are homonyms even with the pitch accent. It's not at all like in English where if you string together commonly-used sounds, you end up with mostly gibberish.
Because of this, if you slur a single vowel and it gets misheard, someone might hear you as saying a completely different word.
Japanese also does not have stress...it is a "mora-timed" language, meaning it consists of syllables in an equally spaced rhythm.
If you try to speak Japanese and mispronounce vowels by shortening them, Japanese speakers will not always hear the words the way you intend them. For example, I frequently hear English speakers mispronounce Japanese words shortening "a" into the schwa. But native Japanese speakers will sometimes hear the schwa as closer to the "o", depending on context. This can lead people to miscommunicate.
I also think the key or "interesting" characteristic here is not whether or not languages have a schwa sound, but whether or not things reduce unstressed vowels to a schwa sound. Mandarin Chinese has a staggering variety of vowels (far more than Japanese) but, like Japanese, it has tons of homonyms and you need to distinguish all the vowels in speaking because hearing a single vowel differently, will usually result in it sounding like a completely different word. There are vowels in Mandarin Chinese that are very schwa-like, including neutral vowels where you basically leave your mouth in the same shape used to pronounce whatever consonant they follow, and also ones that occur in dipthongs that are very close to the sound of the schwa in English. But they don't play the same role that the schwa does in English.
Also keep in mind, languages can shorten vowels to sounds other than a schwa.
For example, Turkish uses vowel harmony, but it isn't perfect in the sense that like, when you put a suffix on a word, the vowel gets reduced in a certain pattern. Turkish does have a schwa sound too, it is denoted with a dot-less "i", i.e. "I/ı". And sounds are sometimes reduced to this sound, but not always. For example, to make "Adam" into the direct object using the accusative case, it would be "Adamı", so the "a" vowel creates a schwa sound after it in the vowel harmony. But the "e" vowel creates an "i", so for example "Ekmek" becomes "Ekmeği", and the same pattern plays out with other words, like "Yer" (eat) becomes "yerim" (I eat).
If you start learning Turkish, you'll realize that the system of vowel harmony is pretty intuitive...when you speak fast, certain sounds wouldn't get distinguished, but others would.
Basically, I think the "interesting" question here is a little broader. Just having a schwa or not, isn't the same as reducing vowels. Languages can have no schwa-like vowel (like Japanese) and not reduce vowels. Or they can have a schwa-like vowel (like Mandarin) and also not reduce vowels. And they can have a schwa (like English) and reduce vowels to them a lot. Or they can have a schwa (like Turkish) and reduce them sometimes in a sort of predictable pattern in certain circumstances but not in the exact same pattern as English nor as often as English in situations where we would.
And I think, whatever happens, it comes down to the language needing to be understood as it is spoken.