Because they can’t read music, and they’re poor at singing by ear.
Because if they can read music, the music often isn’t supplied for them (just the words are printed), and parts are virtually never supplied for those of us who read a part other than the melody.
Because the instrumentalist, especially an organist, will not play a clear melody line, but instead, embellishes the accompaniment making it difficult for those who sing by ear to pick out the melody.
Because most of the cantors are not music conductors, and the congregation is uncertain of entrances, cut-offs, etc.
Because often the instruments, especially the organ, do not accompany the congregation, but lead it, causing the people to get lost.
Because they haven’t been taught how to sing in a head voice which would enable them to reach those “high” notes. (BTW, a D an octave + 1 above Middle C is NOT a high note.)
Because the songs and hymns are played at an improper tempo (either too fast, or more often, waaaaay to slow) that does not allow breathing in the appropriate place in the music, resulting in a “breathless” or “airless” sound that is very weak and volumeless.
Because in our current culture, corporate singing is simply not done in any circles nowadays other than “church” and it feels uncomfortable for us. Instead, in our culture, even in “Christian” culture, singing is done by professionals, and the rest of us attend “concerts” or buy CDS, and merely listen to others sing.
Because they have received the teaching that singing is to be contemplative, not jubilant, and that to “sing out” is to call attention to themselves rather than Jesus in the Blessed Sacrament.
Because they have received the teaching that “reverent” means “soft.”
Because they are old and do not have the strength of voice anymore.
Because they are concentrating on praying or other devotion to Our Lord during the hymns, instead of being “in the moment” and devoting their time exclusively to the singing of the hymn.
Because they are taking care of babies and young children and can’t hold the hymnal, too.
Because they are babies and young children and don’t read yet.
Because they don’t understand the words to the older hymns and have difficult time fitting them into the melody (or they are snickering about “ethereal lays”).
Because they have condemned the modern hymns as heretical, and are demonstrating their objection by refusing to sing in protest.
Because they are not actually participating in any of the Mass, but are just fulfilling their obligation to prevent themselves from going to hell. Hymns and spiritual songs are so far-removed from their “real life” that they simply don’t even bother. (But at least they are at Mass and the Holy Spirit can give them some graces.)