They're massive because they're made to be that way. Technology transition takes will and backbone. If that's missing, there's where $25M gets spent down the drain.
I agree they generally don't go well. Because they're run by weak Directors and VPs who are 2nd-guessing IT and getting in the way.
Plant differences are all well and good. But so is "IT not collapsing under 20 years of cruft". It costs something to bring all the plants in line. But it also profits e.g. they're still in business after 10 years.
Here's how the money part went. Once the tracking went online, they realized (region by region) they were losing $1M per month. 2nd month: losing $0.5M. Third month: break even. 4th month: 1M in the green.
Why? Because they now knew (for the first time) what the hell they were selling, how much to whom, and for what price. The VPs quickly became totally addicting to this instant-knowledge web API and used it to turn the business around.