In our Latin Mass in our city, the congregation never sings the Gregorian chant or other chant. It is always sung by a choir (schola) of men, although there is one young lady who will sing with them for certain pieces. I know little about various chants, but I know that the music director of the Latin Mass parish DOES know about chant, and he knows which pieces are appropriate for a female singer.
So I would never expect a congregation to sing Gregorian chant. Of course, I’m a convert to Catholicism (2004), and perhaps in the past, the entire congregation joined in on the Gregorian chant? Is this the way it was? Did the congregations join in the Gregorian chant back in the day?
But the Latin Mass did not specifically call for “active participation by the congregation” as you have pointed out that the OF does call for. So I agree with you that it is a matter for concern when the congregation in an OF Mass doesn’t sing. I certainly get frustrated when I see so many people just staring into space, not even opening the hymnal (we have hymnals, not missalettes).
The congregation does sing plainchant, yes, which I think is very ugly even when it is done well and especially when it done poorly. So monotonous, like a drone. Most people sing it in a lower register because they’ve never been taught how to sing properly, and most people in our part of the U.S. sing diphthongs which are grating and horrible.

They don’t sing through their vowels–so awful. But to be fair, they do the same thing with hymns! It just sounds worse with chant.
My personal opinion on why Catholics (and many Protestants) don’t sing is that they were never taught to sing or read music in school. In our city, during the 1980s and 90s, and even into the 2000s, music was eliminated or greatly cut because everyone considered STEM to be of utmost importance for children and teenagers.
And in the schools that still offered music, traditional singing of songs was curtailed in favor of offering “ethnically-aware” music. In our city, many of the schools offered a “hip hop” class, and the kids would perform hip hop at their programs. Or rap, or jazz (yes, for CHILDREN–aargh!), or various world styles of singing, especially African. Yes, I think that elementary music education should incorporate ethnic music, but NOT at the cost of training young children to read music and sing correctly.
BTW, I know that young children can and do learn to read music. For years, I played for a children’s choir offered by our local music club, and children as young as 6, from all economic backgrounds, picked up sight-singing within a few weeks or months after joining the choir. Music teachers use several methods to teach this skill, and it works!
Schools are starting to see the error of eliminating the arts, and we are seeing many of the music classes re-instated in our city. But we’re still seeing a lot of the ethnic music instead of traditional song-singing, and so i suspect we will continue to see some very bad singing in our churches.