The other part is that management/PMs usually care about deadlines, not work effort (duration in man-hours, story points, or whatever).
Allow me to indulge in a fictional narrative:
You are handed "high level" (a.k.a rooted in the fuzzy non-technical alternate reality that is product management) requirements resulting in known unknowns and unknown unknowns. You're junior and naive and you put real tech-borne effort numbers in front of each task; or you're experienced and you put O(t) numbers on the first ones and random but high numbers on the second ones.
Anyway, the estimates end up landing past the desired deadline, management mutters TLAs such as "OKR" and whatnot, you'll have to justify why it doesn't fit, so you communicate work effort.
You are summoned into a retroplanning meeting to cull out stuff based on work effort so that it fits management's expected deadline, and try to pretend you're doing agile when it's all waterfall Gantt in disguise.
Project starts. Implicitly man-hours become wall-clock-hours, irrespective of other tasks such as CI/tooling/dependency breakage, code hygiene, or customer support escalations; your effort estimates became t{project start}+effort == deadline. You are now held accountable for the latter based on you communicating the former.
Time is now the scarcest of resources, you do your best, conjure magic, work extra hours, take shortcuts, and make it to the deadline, sacrificing body and soul. In a cloudy spreadsheet checkboxes have been checked, management is happy. Deployment is smooth but soon corner case bugs and escalations start trickling in. Management is either oblivious or not so happy after all.
Time for a new quarter to start. You know issues borne out of past quarter will rear their ugly head, and there's much tech debt to repay before interests quickly accrue towards tech bankruptcy. You're junior and swear to yourself to not make the same mistake, and you start inflating numbers; you're experienced and you realise the whole number dance is a sham as they don't matter anyway, but you still are required to do the dance because that's how the game is played, so you turn Scotty and just throw outrageously big numbers up in the air and just make sure the deadline is not too ludicrous, still hanging to the hope hoping that someone, somewhere, has seen the light of reason and built a company around it, and maybe you could go to these greener pastures; you're very experienced and you realise the only winning move is not to play, so you quit (not the job, the _industry_) and move to a remote plot of land with literal green pastures and a small cabin mountainside to raise sheep, or cut wood, or whatever, where the smallest time unit is the measurement error of eyeballing the zenith-relative angular position of a giant fireball 8.3 light-minutes away.