>>512118642 (OP)
Because it is a form of disengagement.
Boomers are extremely socially inept. They were awful at actually making friends, they instead forced familiarty by enforcing arbitrary social norms and ostracizing anyone who falls out of line. This had the side effect of making most boomers unable to think outside the box (which is why they value this skill so much in companies) or be unable to go against current trends.
When boomers tell you off, they expect some huge reaction, because in their minds they are a righteous guardian against the non conforming evil that is you. They expect you to either cry, grovel, get angry, get sad, etc.
If you do none of these things, and refuse to entertain their bullshit, it drives them fucking insane.
Boomers cant handle someone who will neither react nor respond while still acknowledging their presence. It means the zoomzoom knows they are there, and they're yelling their old man words, and he doesn't care. They have no power. They're just a balding coward from a generation where any job would guarantee a lifetime of success. They never learned new skills. They didn't actually fight in wars like their predecessors. Boomers are the most entitled generation of all time. They believe everyone (from their age bracket) is entitled to a lifetime of a well paying job even if you have no skills (hence the all you need is a firm handshake meme). They also believe they are entitled to your attention, respect and admiration.
The zoomer stare shatters this illusion - it reminds boomers they can't have everything they whine for and that the times have changed, and they either they adapt like every generation before or since has done, or they will perish.
Gen X, Y and Z have no problem accepting that they will have to job-hop and continuously learn new skills until they die. They speak multiple languages and are digitally literate. Boomers struggle with the simplest of tasks and usually require a team of X's and Y's to pick up their slack