Why

Usually amateur choristers are people who love music, but they are not always singers.

I believe the lack of familiarity with using the vocal instrument is the primary cause, in addition to an untrained ear, but it's not always like that (I was part of a choir and I can tell you that I met people with an excellent ear even if they didn't know music).

This is not the only reason that makes the performance less appreciable; attacks, crescendos, decrescendos, etc., are all elements that have an impact, but beautiful things can be done if there is passion and commitment.

Natan, why are you asking this question??

Opinion:

choirs, even professional ones, are often out of tune because choir masters consider intonation to be a secondary matter. Being perfectly in tune is an extremely difficult thing; one achieves it through SPECIFIC studies. I once had the chance to sing in a choir specialized in Renaissance repertoire, the "Costanzo Porta" of Cremona, a fine choir, and I assure you that the attention to intonation in a choir performing that repertoire is bordering on paroxysm. Everything always and anyway stems from the perception of harmonics. If we remain convinced that the voice performs one note at a time, even in a choir, then nothing interesting will ever come of it... more voices in unison or in consonance naturally amplify each other, they go into resonance, and therefore if 2 people sing the tonic and dominant WELL, at least the octave above is CLEARLY perceived. Anyone who hasn't experienced this sense of consonance probably hasn't fully understood how in tune one can be in a choir.

Obviously, this type of study puts vocalization in the background. The sound must be clear, not vibrato, not nasal (vocal resonances in a choir should always be bright, never cavernous). Paradoxically, amateurs have an advantage in these matters.

And indeed, excluding specialists in Renaissance repertoire (who often cannot truly be called amateurs), the most in-tune choirs in Italy are those of the Alpini. Male choirs of people who have never studied singing.

And the least in-tune choirs are, notoriously, those of the opera houses...

I agree with Thallo; as soon as I read the first post, I thought, proposing a parallelism with the instrumental world, of the difference between tuning and intonation.

In my opinion, the most annoying thing is precisely the lack of care for harmonics, which create strange and unpleasant resonances.

The missed note happens and is easily identifiable... a "random" mass of harmonics, beats, etc... creates an inexplicable feeling of discomfort for a listener.

The issue of harmonics, as posted by Thallo, is very clear and I fully agree with it.

Furthermore, I would add that it depends on the type of repertoire: if, for example, we had an alpine choir sing a Mendelssohn motet, perhaps the result would not be entirely satisfactory...

In reality, I believe that intonation also depends on some simple basic technical measures that any "amateur" can learn: breath support, opening of the throat, and forward sound emission.