Because APM matters.
Because Hotkey muscle memory matters.
Because Spatial awareness matters.
Because Content experience matters.
Because Resilience under pressure matters.
These are all things that non DPSing healers have a tendency to struggle with. Deny it all you want, it's a simple fact.
It's not just as simple as pressing Regen or Cure 2. They can't handle more DPS buttons because they don't have the APM or awareness and oversight to utilise them effectively. And if they don't have good enough awareness or APM, that is going to hamper their ability to heal as well. Go look through your 24 man co healers over the last few years, it's right there in the logs. Look at the healers that have a low CPM rate, odds are they are the ones that are failing at DPSing, failing at healing and failing at mechanics. All 3 things go hand in hand. As such, holding some kind of belief that a healer who isn't able to handle doing DPS with some vague level of complexity will be more than fine at being an Osu superstar once the red buttons turn green is insane to me.