Each and every one of these systems suck. Sometimes they suck in creative and extraordinary ways. But, one thing you can count on is that there's a real reason that they suck. It turns out that the real world of business is complicated and messy. So, as you say, businesses build their own solution on top of these systems to simplify it for their users while keeping the capabilities of the underlying system intact. Of course, nobody is ever happy with the end result - it's always just the least terrible option.
On a slightly related topic since this may be seen by folks who can benefit from this information.
A lot of "Open Source" systems these days are not what you could honestly call open source. Odoo is the example I'll use, but what I'm about to say applies to a lot more than just Odoo. Most of the organizations behind these solutions are predatory. I've seen features get taken out, hidden, intentionally broken, or have fixes released that are only available in the 'pay us' versions. [0-1]Odoo's accounting functions are one such example.
[0]https://www.odoo.com/forum/help-1/question/is-accounting-fea...
A close second is Vienna Advantage. IIRC the development actually came from former SAP devs who have real-world exposure to the problems ERP is supposed to solve.
I know a few colleagues who enjoy that kind of work, slapping a much better and intuitive UI on top of their own services integrating with SAP, Salesforce and so on.
You improve overall productivity and can show that data comparing how much faster someone using your interface is versus the likes of SAPgui, etc.
ERPNext is not open core like Odoo.
Disclaimer: I am the founder of ERPNext
I think rushabh's point was that the Odoo model is an open source core with closed source paid components, whereas ERPNext is fully GPL'd.
ERPNext monetize via a SaaS model, but you are free to self-host.
One system had three completely different, supposedly equivalent, client UIs. Each was equally undiscoverable, and had its own set of arcane, non-standard and inconsistent UI conventions that was completely different fron the other clients. So even if you knew how to enter a certain record in one client, your knowledge was almost wntirely useless with confronted with one of the other two. I am not even getting started on the server implementation and the myriad of bugs in the system.
SAP is not the only one; ever tried the enterprise software from Oracle? Or the open source ERPs/CRMs? Painful.
But everything has API's these days, so lot of departmental work is done on top. Small budgets so they have small, simple LoB apps made on top which are 'idiot proof'. These apps are very localized though; they cannot translate to other verticals, or even other companies, or even other countries because they are tailored exactly to how that department works.