Because it is a case of "you're damned if you do or you're damned if you don't."
I remember back when I served as a younger blinkered elder I very much deferred to the older and - or so I mistakenly believed at the time - wiser more mature elders.
Anyway we had one lad who had formed a relationship with a wordly girl and had been seen by a couple going inside her church. (For all we really knew he may have dropped her off and been using the toilet facility inside the building!)
He was invited to come and talk to the elders but he turned the invitation down. The committee met together anyway, reviewed the alleged evidence, and disfellowshipped him in absentia.
He later said nobody had even told him why he was being disfellowshipped. He was asked to come to the next mid-week congregation meeting, which he did, and was as shocked as everyone else when the announcement was read out.
So it doesn't really matter if you present yourself or not. Minds probably have been made up in advance.