Because every customer needs different features. No one wants to make their workflow follow what the software dictates, they want the software to support the workflow the business uses
Edit: There can be risks when an ERP supplier fundamentally fails to understand your business model - SAP managed to do this with a former employer of mine which led them to be shown the door.
Far too often.
Sometimes we fight a bit to convince our clients to actually take advantage of what a modern system can do for them.
Sometimes we have to give in watch the horror unfold as they use our services to hackily re-implement something that is basically a copy of an old desktop app that was a copy of an old mainframe app that was just an automated version of a paper-based system…
It wasn't a criticism of the SAP product - just that they the sales team constantly got a a basic thing incorrect in a rather dogmatic way.
I would actually go further and say that enterprise ERPs are full-fledged development environments, often with custom languages and code editors. Quality varies wildly, though.
The fact that some of them are able to handle the processes of some companies out of the box is almost accidental.
There's flexibility in ERP's, for sure, but it's not necessarily accessible to the people that use it.
I once had to implement a "screen" to physically divert a list of devices with certain serial numbers. Basically: send a notification whenever one of these devices showed up at a loading dock.
The most logical way to do it, which I naively considered first, was to set-up something in one of the ERP modules which is specifically focused on "material movement". After some tedious email exchanges and a phone call it turned out that it was, in fact, "possible". The catch was that it would take WEEKS and involve an expensive Oracle consultant requisition.
I put an end to that and instead had a junior implement the solution in a downstream application (which we actually develop, own and control), in about an hour. It was worth it even though it meant the stuff left the loading dock and ingressed into the building, requiring some additional physical "moving-around" hassles.
It's not all gain.