Most current companies hold a significant amount of legacy in their architecture. The reason is simple for their case; they operated in the past decades. With every new system implementation, some parts remained in their architecture. But for newly founded companies, the use of legacy architecture is inexplicable. The only reasonable answer is they hired their IT staff with legacy knowledge!
Today implementing a complete system could be only a temporary solution for bridging gaps between existing and wanted processes.
In the past, I gathered knowledge about why an integrated ERP system is a good choice! The ERP system was never full-pledged.
The companies that implemented those solutions have already started to disintegrate the company.
Everything was integrated from the network through identity management to the ERP. If I take my historian hat, I think the only one that integrated the Novell Netware ecosystem with Magic.
From the Windows Gui era, we started disintegrating our systems into multiple ones.
Nowadays, none of the systems are fully integrated. The cloud/SaaS doesn't help either. With the multiplication of the systems, we increase our security risks. The reason for these trends is simple. Many companies create fortunes solving the problems they created!