The insult works because it diagnoses rather than attacks: your take is so warped by internet immersion that only physical contact with nature can fix it. It is the modern descendant of "go outside" and "log off."
Self-directed use is just as common — "I've been arguing with strangers about a TV show for 3 hours, time to touch grass." It also anchors a genre of digital-wellness content, and "grass touched" is the ironic achievement for going outside. Related escalations include "touch grass immediately" and prescriptions of increasingly extreme outdoor activity.
