And yet ... every ERP installation I've EVER seen
is a custom ERP application, with maybe a standard "big" database at the center (not in the case of Odoo, or MS Dynamics that use standard non-ERP databases, or at least databases that don't usually work in ERP systems, so ERP without a big database in the middle exists). These were developed by teams of consultants and maintained by multiple teams of sysadmins, SWEs, DB admins, ...
The supposed extra features are usually 20-year-old disasters. For example, one thing they all have and are very proud of is a standard UI system. That means the "same form" works on Windows, on the web, on phones (these days), with a centralized auth provider, ...
Except to say that ABAP GUI sucks is like saying first hammering a spike into your own skull and then having it pulled back out by a bull is a suboptimal experience. The question that joke screams "why would you do it like that" is one you'll be asking a lot when using these ERP systems. In case there's any doubt: their UI most definitely does not really work on mobile without extensive changes ... also known as "development". And yes it's been improved and is better. Which is to say at one point I believe Oracle had a "mobile UI" which was an android "RDP client" (sending screens pixel by pixel) connecting to a windows server which ran the windows binary of their UI. Even required per-user windows licences. I don't know how they did it, but it actually had windows 3.1 style scrollbars on the phone display. These are improvements in the sense that HIV is a big experience improvement over gonnorhea.
And you might think "surely that means the form is going to be very responsive !".
Heh.
These installations require DB admins, require SWEs, require managers, oncall, ..., require colocation/dedicated/cloud costs (may God help you if you're paying cloud prices for your DB, and whilst I agree admin is easier it's not so much easier that you don't need a DB team). Oh and I've never seen a SAP ABAP installation go 6 months without a consultant getting hired to solve some issue the company couldn't solve.
Just about the only really unique thing about ERP software is that the code for the "applications" lives in the database itself (MS access, or Dbase used to do that too). This is also just about the worst thing about them, as it means you don't get integration testing, or standard version control, or files, or ...
Yes, I know, somehow Oracle has convinced every MBA to do this rather than pay 5 SWEs to just make them PHP apps. And no, I don't mean those PHP apps will be worse, they'll be way better. It's standard so you can get consultants to attempt to fix the worst disasters.