So, back in the day there was a guy named Blackjack who posted various thoughts on Shadowrun, GMing, etc. Really good resource, very insightful even now with 5E out (he GMed mostly for 2E I believe).
Anyhoo, he wrote a couple very good pieces I think are pertinent to the question at hand.
http://web.archive.org/web/20010816174416/http://archive.dumpshock.com:80/bjcorner/ShowBJ.php3?page=panther.htmhttp://web.archive.org/web/20010617122409/http://archive.dumpshock.com:80/bjcorner/ShowBJ.php3?page=hurt.htmThe first is about heavy weapons (since you mentioned that obscene damage code) and the second is about getting hurt, wounds, etc. But the TLDR is this: in the real world, it us EXTREMELY easy to injure someone, if not outright kill them. The gun doesn't need to be big, the knife doesn't need to be long. You take a tiny little .22 calibre handgun, press it to someone's skull and pull the trigger, they're dead. Or that little 3-inch Swiss Army knife doesn't seem too bad, but you jab it into someone's neck and good night.
And as people mentioned above, since Shadowrun tries to be more on the realistic side, that's a good thing. Because the best way to survive a bullet is to NOT GET SHOT. You ever see CCTV footage on YouTube of a police shootout? People run and duck for cover, or hide behind things, because the best kevlar in the world still won't help if that one lucky bullet goes up your nose.
The other side of that is, the (meta)human body is a remarkable machine, capable of surviving and recovering from all types of horrible injuries. That's reflected in the damage code of most guns not being bigger than the standard health pool of 8+body/2. The average human with 10 hp can survive a single 6P damage round. Even a grievous 8P-10P gunshot, they have a good chance to survive if they get immediate medical attention.
A lethal system means getting into a firefight has real danger and consequences. Ice personally seen too many games where the tanky character is facing down 3 guys with machine guns and their first instinct is to draw their own gun. In real life, that guy may fight back(say he's military or police, most regular people will just soil themselves), but he's sure as fuck going to seek cover before he does so.
Because trying to stay alive in a dangerous world is smart, and Shadowrun is all about rewarding smart play.