It's an emotionally loaded topic for everyone, people might feel uneasy to touch the topic, they might fear it can happen to them (very likely, no one wants to be the BS one day), they just do not know how to approach it respectfully and are afraid to slip and cause you pain.
Consider as a default is very common for people to think "If it ever happens to me to be the BS I will surely leave the WS and never look back"
Many of use were utterly convinced of that too.
Still we tried to R.
People who were never there might see this as impossible to understand, they may have deeper sympathy for you, perhaps even projecting resentment towards your WS as proxy, and still feeling awkward and avoiding the topic because they are unsure why you R and how to express their concern without hurting or offending you.
Maybe also you do not like bring it up, so it is a topic 'hanging there' but taboo, hence they might find more comfort in just being around you (or him) and avoiding what they perceive as landmines.
I am living a similar situation with a close friend of my WW:
She is also my good friend, she is in her early 40, 3 years in a relationship with a serial cheater, we are losing count on how many times she caught him in the act, they keep splitting then he manipulates her back, sweet at the start, then shifting the blame onto her, then restarting the cycle (which I believe he never stops, he is just a good manipulator).
Nothing to say, she is destroyed going through all the trauma of being the Betrayed Partner.
Ironically, being the good friend of my Wife, she was also present to witness (and 'support') both of her affairs.
I do not resent her for that because she is a woman, she is my wife's (gf back then) friend first, and that's just what friends do.
So she talks with us about her pain and relationship - let's don't linger on the irony that my own WW speaks with outrage how that behavior is unacceptable and she should forever leave him. I agree, but I can't help but laugh at her cognitive dissonance here - she tells how much it hurts and she listens to my advice particularly (because she knows I used to wear her shoes).
And still, even while suffering the heat of it, she avoids and it's reluctant to acknowledge or touch the topic of our betrayal story. Even if we are friends, even if she was there, even if she know everything, she can talk about her betrayal but, her friends (us) betrayal is a taboo topic.
My wife last time we met brought up her own betrayal as an example openly while we were discussing hers. She changed the subject immediately, visibly in discomfort.
I do not feel this barrier to talk openly with her about what she is going through, even when she brings her WP at us for dinner. But she feel her friend's one is taboo.
It's the only example I can bring because is the only one I experience.
There is something psychological at play, it would be interesting to understand what and how to normalize it.