From an earlier comment, but this is why:

"For every $1.00 spent in manufacturing, another $1.81 is added to the economy. That is the highest multiplier effect of any economic sector. In addition, for every one worker in manufacturing, there are another four employees hired elsewhere. (Source: NAM calculations using IMPLAN.) - See more at: http://www.nam.org/Newsroom/Top-20-Facts-About-Manufacturing...

http://www.nam.org/Newsroom/Top-20-Facts-About-Manufacturing...