An imagesetter is indeed very much like a beefed-up laser printer, but they are certainly not on the same playing field.
In full colour offset lithography, absolute positioning of dots, and absolute repeatability are critical - output must be exact. In addition to accuracy and repeatability, the shape and sharpness of an imaged dot has to be very precise: overlap or gaps is not tolerable. Do all of this at 3000 dpi, and you can imaging the level of engineering required to pull this off, and the considerable maintenance to keep it all working.
The innards of a classic Linotronic drum imagesetter are gorgeous: chilled air-bearings for the laser feed screw, automatic registration punching, precision film feed, tensioning and vacuum system, etc., etc., I was responsible for the care and feeding of several of these beasts in my pre-press days; fond memories