Bottom line, it's a byproduct of having a serial comic, that's been perpetuated over decades. If you're going to keep the same villains around (and of course they are, because if you try and change anything within a comic mythos, the fans lose their fucking shit), then they can't die, and you have to keep coming up with ways for them to escape punishment, and get out to do bad stuff again....so the hero can stop them again....so they can escape again...so the hero can...eh, you get the idea.
Yes, it makes for some annoying trends when you look at it long term "Batman doesn't actually improve anything in Gotham, he barely keeps things from getting shittier at best", but that's just how it is. I mean, if someone actually tried to...you know..end the Batman comics, by saying "Ok, so after 60+ years of fighting crime, Batman has finally broken the criminal choke hold on the city, and the citizens of Gotham are free to live their lives in relative safety from crime." If someone tried to officially do that, it would end a ridiculous amount of money for DC, and the internet would explode with all the nerd rage from the fans, who can't stand to see anything change with their beloved character. It's basically financial suicide for DC. So no, nothing will ever change with comics, they will always get away, and the hero will always not kill them, because if he does kill them, again, the fans will flip their shit about "Destroying Batman! You raped my childhood!" blah blah.
Just deal with it. As illogical as it is, given the long term representation of these villains, to let them live, it's just not something any sane writer at DC is willing to do with a comic line. And even when they do have the guts to shake things up, eventually a new writer will come along, and retcon what they changed. "Oh, it wasn't actually Spiderman that died, that was a clone of spiderman, the real spiderman is just fine see?! Keep buying these comics!!" And again, you go back to status quo.