Like I said: because I suspect nothing and I'm happy.
Nobody ever wants to accept this. They think I'm trying to avoid the stigma of divorce, or I think I'm unlovable, etc etc. That wanting to keep my happy life isn't a reason, or not a good enough one.
The reason is because I'm happy with my life and I wouldn't appreciate a total or near stranger barging in and overturning it. And if they were awful enough to do it anonymously, so that I couldn't even assess the source and because they think it's worth every cost except to themselves...I can't even begin to describe the contempt and anger I'd feel.
Do you feel this same questioning for people who discover an affair and decide to stay?
What it all looks like to me - the need to tell even when these people are practically strangers, the invalidation of my reasons - is a veil for people who think cheaters should always be punished at any cost. So a desire to pressure people to do what you think they should do rather than what they actually want to do.
Perhaps it is a personal failing in me that I wouldn't want to know, but I don't care. It's my life and my decision and I'm not required to live it to someone else's beliefs.
I might feel differently if I suspected something, was unhappy and someone who was reasonably close to me spoke to me, openly and honestly, and was there to be non judgemental and supportive afterwards. Maybe. But while I'm totally happy, from someone who's nothing to do with my life and is only going to sod off afterwards? No way. Leave me and my family alone.