M&A (mergers and acquisitions) is a big factor - for most large corps it happens all the time, you haven't finished integrating all your acquisitions before you get new aquisitions or get acquired yourself. So if you go the bespoke path, you have to continuously migrate departments and their data off of SAP or something like that. And when you get acquired, you'll likely have to migrate to SAP or something like that - and, more interestingly, you may come to a situation where you need to migrate to SAP or something like that before an acquisition to remove a potential risk factor for the acquirer you're targeting; that's going to cost a lot but it's worthwhile given the expected M&A money.
True! But than, when Amazon started, there was no solution for their problem, e-commerce with an integrated logistics software suite, available. Which gave Amazon a huge advabtage over competitors. That changes since a couple of years if you ask me. Which could be a problem for Amazon in the long run. Very long, so.