I don't get the logic here, why is it so important that someone show up in court? Proof is proof. If the case relies on the defendant admitting it, then you don't have a case...
Because the stuff that happens in the op is the extreme minority. Imagine a rape victim pressing charges and having to relive the trauma every time a hearing is held. Now imagine the accused never showing up to trial because fuck you I don't have to show up if I don't want to. Then the judge says "ok we can't do anything because he didn't show up, come back in 30 days for another trial".
Then repeat that scenario every time over and over with no repercussions for the accused for never showing up to court. The victims then relives their attack every time thinking this time is the time they finally get justice.
That's why.