This is a distinct American trait. I was born in Spain to a Spanish mother but lived here for 20 years. Here’s my take:
People say they’re fine because American culture is surface level, self-serving/insecure, and punitive. It’s also paradoxically obsessed happiness. I say paradoxically because American values include exclude everything that leads to real happiness.
If you’re not fine, well then, it’s your own fault. (Punitive)
Because many people are too fragile to hear other people’s reality. They never learned how to detach from other people’s drama, so they get stressed out. Or they get irritated if you tell the unpleasant truth.
Because people will take your pain and use it against you, especially if you’re marginalized. Most especially if you’re Black. (Self-serving/insecure).
If you got to other countries such as Spain or Russia, people will answer honestly. Maybe even overshare.