Running under 20 minutes for a 5k takes talent and more of it than a lot of you realise.
I know people who absolutely bust their guts for sub 21, and I'm talking 400-mile repeats, speed work, occasionally doubles, weight training, the lot.
You're all too harsh on the slower people. Fair enough, you can't get sub 16 or sub 17 or even sub 18 without a lot of hard work but those times take a fair bit of god goven talent (cardio efficiency is mostly genetic) and I've seen people with the same volume of training as a 16 minute guy hit much slower times.
There's a reason ~20 minutes for guys and ~24 minutes for girls wins the local 5ks and that's because those people train harder (a lot harder) than you guys think. But alas, vo2 max is mostly genetic and most people here don't realise that only talented people score beyond 43(female) and 50(male).