AZ,
I don’t know why they are not paid for time invested for matches that don’t work out, and it’s possible that the trend should change here- I’m not qualified to be the one to change this, although what I would do in my personal life is what I believe to be, proper.
The time-honored “trade” of shadchan has always been paid for a successful shidduch, the amounts obviously keeping up with “inflation” with variations in and across communities. Hakoras Hatov as shown in a token gift, even monetary, is simply mentchlich for efforts invested by a shadchan that do not result in a successful match (hakoras hatov expressed in conjunction with the shadchanus gelt is I’m sure always appreciated as well!) and I’m sure people with appreciation often do so in some form or another.
If your suggestion takes off in some form or another, I would think that is a positive change.