Because there are huge downsides that outweigh the benefits.
1) It they had those buses get off the highway, go into Don Mills, and get back on the highway, i) it would add 10-15 minutes to their trip each way, a lot of people have 30-45 minute trips on those buses currently so this is a huge increase, ii) it would prevent the buses from using the HOV lane southbound for the most congested part of their route, and make it much harder to use it northbound as they'd have to cross 5 lanes vs the existing 2 and have a shorter distance to do it.
2) I'm not aware of significant demand on the southbound buses for people coming in from Richmond Hill, Aurora/Newmarket/Barrie, Markham/Stouffville to exit their buses at Don Mills. I think most of the passengers are headed to the downtown area and can get to their destinations much more quickly via union. If so many people needed to get to the Sheppard Line, I don't think they'd be going all the way to Union in heavy traffic to do so, they'd be taking other transit routes or driving--the people on those buses want to go to Union.
3) Regarding it acting as a "relief line", there is often no room on those buses southbound at that point, and they are similarly often full leaving Union northbound. A lot of passengers trying to get to RH/Barrie/Stouffville line destinations would be left behind at Union because people who could have taken the subway took their seats to go one stop within the city, and coming southbound the bus would lose 10-15 minutes, plus time lost due to not taking the HOV lane, for no reason since almost nobody would exit at Don Mills leaving no room for boardings. I've had to wait for a second bus at both Aurora and Unionville before, at Unionville I was lucky to be first in line for the second bus as it only took on a few people leaving over a dozen passengers having waited for the first bus behind, plus those that arrived for the second bus.
So, I actually have to disagree that there would be "so many benefits", I see virtually no possible benefits for a huge negative impact on thousands of commuters who take those buses every day and don't have feasible alternatives, except possibly driving and adding even more congestion and pollution to the region. People near Don Mills and at STC have alternatives that they've been using for a long time: the subway; it may take a little longer and be a bit more crowded, but it's nowhere near as bad as making bus service useless for people as far as Barrie and Stouffville.