It's because you're supposed to customize them, not use as-is. We've had a lot of happy customers. Some send us gifts! But for the first year or maybe even couple of years, you probably pay more to your partner for implementation, customizations and advice than to the ERP developer for licensing.
ERPs aren't for every company, different ERPs work best for different companies and different partners themselves have their own specializations. The one I work through (used to work for, but now I have my own company and just contract for them), does small to medium sized production companies. Think 5-200 employees usually. The ERP we work with is meant to cover every imaginable use case - which is why it doesn't have enough depth. We add a bunch of stuff that isn't there OOTB, sometimes remove things in default modules, etc.
But first you NEED an ERP partner to make the most of it. At ours the CEO is also the biggest salesman. He's not afraid to tell you if he doesn't think it's a good fit. A bad partner will still try to sell you and that's going to end up in disappointment for everyone.