It's maybe worth mentioning that the obvious reasons almost entirely have to do with the things you'd hit in the air, not once the bullet descended. If everyone on earth suddenly shot upward at around 80 degrees from the ground, you'd expect approximately zero casualties on average from the bullets falling and lethally striking surface objects (there would be a number of welts and bruises as well as minor property damage, but even that is less likely than you might suspect).
Do you have a citation for that? I can't find any numbers on people getting hit by falling bullets, let alone fatalities. Things let celebratory gunfire don't seem to be comparable since people tend to congregate outside for celebrations resulting in larger groups of potential unintentional targets.
https://jamanetwork.com/journals/jama/fullarticle/201323
Celebratory gunfire incidents were included, but only make up 4.6% of the total.
In approximately one year:
>Altogether, 317 persons received stray bullet injuries;
>142 (44.8%) were female, and 176 (55.5%) were outside the age range 15 to 34 years.
>Most individuals (258, 81.4%) were unaware of the events leading to the gunfire that caused their injuries.
>Many (129, 40.7%) were at home; most of these persons (88, 68.2%) were indoors.
>Sixty-five persons (20.5%) died, 18 (27.7%) of them at the shooting site and 55 (84.6%) on the day they were shot.
>Fourteen persons received nonfatal injuries by secondary mechanisms.
Also, injuries and one fatality were confirmed by Mythbusters in [0]. Read the account for Episode 50, which was the only myth to receive all three ratings (Busted, Plausible, and Confirmed) at the same time.
https://www.statista.com/statistics/1284427/stray-bullets-vi...
Yes, it is reasonably established that bullets fired straight up do not regain their initial velocity on the way back down.
But, at less-than-vertical firing angles, they can retain a lot of their initial 4-figure-fps velocity, and how much is very dependent on particular trajectory, wind, bullet composition, and initial angle. Dependent enough that it's specifically outlawed in many states.
So, don't go glibly firing off skyward celebratory shots (over)confident in your understanding of the physics (or implicitly advocating it). You might be willing to take the risk, but no one else is willingly undertaking the risk you are creating.