Moreover, folks that are experts on an ERP typically are an expert either on (a) one type of ERP (e.g.-SAP), or (b) ERPs for a specific domain (e.g.-manufacturing). You don't typically get somebody who knows many different ERPs across many different businesses. During an ERP implementation, you'll usually hire one of type A, and one of type B, and then a whole team of developers/implementers.
Each vendor gives extensive training on all sorts of different aspects of their ERPs, but really, if you'd like to get a handle on what they do, you need to know how enterprises work. And the only way you're going to do that is by working in an enterprise. I've had my exposure to ERPs by building data warehouses that have to read and interact with them. But, my college degree's in management and economics, so things like accounting and business processes were already something I had passing familiarity with. ERPs will make little to no sense if you don't understand how corproate accounting works.
ERPs are wildly complex, and each has about 1000 different packages that you can use to do...I don't even know half of what they do. The best way to get some familiarity with ERPs is to get hired in some place where they'll teach you all about it. This includes training in the classroom and on the job. These are typically consulting firms--there are a few big ones, and there are a few boutique ones that do only ERPs. Also, the vendors typically have implementation teams, so you may want to do that, as well. I will note that ERP implementers almost always are required to travel extensively, so if somebody reading this wants to go into it as a career, understand what you're getting into. Travel is insanely lucrative if you don't mind it, but if you have a family or other commitments, it can tear you apart.