Why are laser machines so heavy ?

The lower end 40w lasers start at around 30kg but I've seen a 60w with legs at 115kg. Why are they so heavy when they are just an empty tin box ? Granted the casing and legs are sheet metal, and you have metal runners/threads, but it still seems a lot of weight ( 115kg is 1.5 times my own body weight ). I ask because if I buy one that weighs 115kg, I'm concerned about being able to manouver it through a door and into position. If I could tip it sideways it would be easier to get through the opening, but I doubt I'd be able to move it much. Are they that difficult to manouver if they don't have castors ?

Any help appreciated.

I didn't find the right solution from the Internet.

reference:
www.laserengraverforum.com/viewtopic.php?f=11&t=12435
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Thanks!