Because it does not need a buff. Don't buff/nerf something what is in perfect spot.
There is weird tendency to buff units, which are fine, becasue they are not OP.
For example.
Brummbar and Bulldozer are far away from being OP and far away from being UP. They are in perfect spot. Just like Ostwind. We don't need killing machines. We need normal units with quite reasonable and constant damage.
If this is the case then halftrack and centaur are overperforming for price relative to ostwind