In quite every culture, expect Western classical one, majority of musicians can also sing., for example, in minyo musicians sings playing shamisen, in Turkey there are saz player/singers, in Eastern Europe there are some epic tales storytellers who play/sing… so
1) Why this is not deeloped in Western culture, like there are not so many pianists who also sing, ecept pop
2) Particularly regarding minyo, in this case, but I can also generalize. There aren't off-tune people in the world, but some people are off-tune. For example, if I tell my mom to sing, she is terribly not able. But suppose that my mom is an excellent shamisen player, suppose in a sokyoku piece, and she has to sing. I think that majority of players (koto/shamisen/biwa/kin family) have to know/learn how to sing… what can they do if they can't for nature? Yeah, it's obvious that you can say that they can't as well… but… they just can't?
For Shamisen Players I think it's important to know the singing part (and mostly also beeing able to sing while playing, even if it doesn't sound super good) to know where you are in the song.
And if the singer or the player make a mistake or improvisation to extend or shorten the piece, they would still know more or less where they are, and can come back into the song together. Such moments are magic to watch :)
I can't give a good answer to your first question. Anyone can sing.
As for your second question, it's pretty much as Andreas says.
For shamisen players, as well as kokyuu players, learning the song's lyrics helps you match your singer (or fill in gaps, or respond to errors).
It is very helpful on songs that are heavy on repetition or have complex rhythms. Even if you can't sing along, you can understand where you are in the song, and avoid mistakes. Echuu Owara Bushi is obnoxiously difficult if you don't know the lyrics, but extremely easy if you do.
Moreover, for min'yo players the hayashi occasionally is sung by shamisen players.
This is the “response” phrase. It's similar to a chorus.
For example, when I perform Kokiriko with a dedicated singer, I will usually join in for ”Mado no sansa wa dederekoden | Hare no sansa mo dederekoden" sung between each verse.
Thank you so much for the reply
No, I'm not sure if anyone can sing, because I hear sometimes people who really can't… why exactly? IS IT JUST BECAUSE THEY DON?T LISTEN each other?
I think people who “can't sing” are trying to force their singing voice to sing in a pitch it isn't meant for, because they feel embarassed of their own natural pitch and are trying to imitate another. I can say this from my own experience.
You need two things to be able to sing. Tone ear and vocal control. If you can play the shamisen and know when you're playing notes out off key and can adjust it, then it's just a matter of vocal control. As long as there's not some underlying medical condition, everyone can learn to sing. Some just have to invest more time to master it.
In answer to your first question… 1) Why this is not developed in Western culture, like there are not so many pianists who also sing, ecept pop….. actually it has developed in Western culture, but you have to compare like for like musical styles. You site 'Minyo' players as performing both skills, however, as minyo is essentially 'folk music', in Western folk music a significant number of fiddle, guitar, melodeon, banjo (etc.) players also sing.