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.

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