Irasshaimase!

I was wondering why the shopkeepers in Tokyo (Osaka is not as clamorous) scream out "Irasshaimase!" at such a disturbingly high volume when you enter a store (esp. at Shibuya 109 and Harajuku's Laforet). I know that it's a formal, polite way to greet customers but I got startled a few times by this earsplitting gesture (mostly Cuties and Sweeties with extremely shrill voices). Sometimes I would hear them mumbling this to themselves even when nobody is around; out of habit I guess, all the years of hardcore training forever instilled. I was at the malls during the annual sale so "Irasshaimase!" was raised several decibals; louder than I've ever remembered. It's like Chinese water torture taken to new heights. I want it to stop but then again I don't; in a demented, sadist way, I love it. It just makes me wanna focus harder on the task at hand which is shopping. Everything fits me to a tee in Japan. One of the best shopping experiences I've ever had.