Me too I asked myself that question many times.
Some can say is because doping. I dont think so. Of course some of those can be doped. But thats not the answer.
Is something that has to do with their level of fitness. But also with their ability to run relaxed and hide everything emotionally.
Everybody has in mind Almaz Ayana race when she broke 10k wr. From the beginning to the end we just saw on her face a relaxed expression as relaxed was her running style, it was an incredible race.
Elite runners and particular east African , Kenyans runners have the ability to run very relaxed at high speed and to not
show anything from their face expression. This particular in race is a great ability. Cos they dont give any advantage to their
competitors.
When you run at a world record pace doping or not doping you just gives all you have, and I dont believe if somebody is clean
after the finish need to collapse to the ground while if somebody is doped after the finish can smile and keep on jogging.
Maybe is true the opposite .
To me it just depends from how fit and trained somebody is and from individual phisiological and psychological values.