Why is the aesthetic of gyms, even the good ones, always so lifeless? Always black and grey and white everywhere. I want fans blowing air in my face at 12 km/h and a simulation of running along a forest path in a magical fantasy world .... or at least some plants.

Because most gyms are designed by fitness coaches, not interior architects or designers. 
Every coach or trainer what I know has their own vision of fitness and the gym look. By the end of the day technically it’s a place of action and the results will you go do what is supposed to be done and you exit. Simplicity and minimalistic approach give opportunity to open the gym if it given in 24 hours. We can always add additional stuff to make it pretty but the most important it’s to have the place hygiene and good ventilation. What makes good place are people.