Because if you write automation this way then you can’t edit volume for the instrument anyone and have to go into some plugin and adjust it there.
Which is how it’s been for ages, but still pretty dumb.
I usually use level automation with the last plugin, but that’s not optimal either because sometimes you may accidentally delete this plugin and all automation work goes out of the window.
I never automate the channel volume. I always automate a volume plugin. It’s hard to see how Steinberg could improve on that workflow without making things unnecessarily confusing.