It's probably also because our linear motor trains are mostly pretty damn old - the Mark Is have been in service for 25+ years, the first batch Mark IIs for 10+ (though I find the second batch Mark IIs, which are now 5 years old, to be fairly quiet).
The newer linear motor train systems here in Japan have far quieter, more pleasant trains because they naturally incorporated advances in the technology. The Fukuoka Nanakuma Line is pretty quiet for a fully-underground line, and the Yokohama Green Line gives a taste of that quiet-ness above ground.
If you guys want an idea, here's a video of the brand new 12-600 series train just delivered to the Toei Oedo line in Tokyo. These 8-car trains were delivered literally a few months ago and are the world's newest linear motor trains. I was in Tokyo recently and I do believe I rode this new 12-600 train one time as I remember one train had these particular LCD screens. Note that the Oedo Line is completely underground (so think future Broadway subway when watching this).
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