A number of reasons;
- Most of the people playing support are actually tank/DPS mains in flex queue.
- Healing is objectively lower-impact than DPSing in OW2.
- A significant number of players do not know how to position properly to be healed. This can result in a lot of support players not bothering with them anymore.
- Tank and DPS mains literally demanded that support mains start DPSing more because they smugly assumed that healing must equate to healbotting. They literally asked for it.