Buying an off the shelve package also buys you an industry-standard business process. The idea is that part of value comes from the process that you have to shoe-horn into. Some integration is expected (and provided for) but if you have to modify heavily, You're Not Using It Correctly.
Also, using an OTS package means less development risk (not every IT dept has 10x developers coming out the wazoo) and it also means that you have a huge reservoir of skilled labour if you need it. Not just for development but also on the business side.
Finally, if you're a big, publicly owned company, you likely have many financial / reporting constraints. The scope of your non-negotiable requirements may be bigger than you imagine.
I've seen people implement SAP in a couple of months. It's possible. It's still expensive and clunky though :-)