Hyperbole. A lot of working with an ERP is developing middleware or custom modules that implement business-specific logic on top of an existing API and schema. It's like saying building a Facebook App is equivalent to rebuilding the entire Facebook Platform.
> So we've dared to build our own multinational and fully integrated CRM and ERP
I hear this all the time, and it's almost always wrong. Engineers and businesses are always convinced they're special and don't fit the mold. That's rarely the case, and the money used on these initiatives was probably better spent elsewhere. Usually when someone thinks they have some sort of special edge case or unique business, it's more often than not they don't understand GAAP/IFRS/IAS etc and how their finances can be modeled.
At the end of the day it's about capital investment. Either you believe you can do it better and you're willing to back it with money, or you realize it's not core to your business and buy something off the shelf because it delivers the best bang for the buck and more importantly lets you focus on other projects that are actually aligned with your business model.
> if you don't know exactly what you're doing
There's the caveat. You either find someone that's worked with ERP's for at least 20+ years, or you build your own over a decade and gain it 1st hand.