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If I could hazard a guess, the reason why so much of the music played in gyms is so bad is because it is selected long beforehand in corporate boardrooms.
My guess is that your gym contracted with some kind of audio "content provider" company to pipe in a selection of music, and that company has several contracts with record companies to play certain songs.
Those songs are not likely to be the best songs available. On the contrary, it is much more likely that they are the cheapest songs that the content provider could get away with contracting for while fulfilling the terms of its contract with your gym.
You might think of it as "the best of the worst", with maybe one or two genuinely popular songs thrown in to make the rest endurable.