For every "We had a great internal platform that we custom built" story, there are multiple "We used a small vendor's product, and it was a nightmare."
Why is SAP successful? Because the general purpose tools (or worse, amalgamations of multiple single-purpose tools) that came before were absolute shit, and SAP is instead only difficult to use.
Progress!
(Also, a TAM that is literally every company, ever)
The people who are hurt by it are not the people deciding to buy it.
SAP looks great for Csuite executives when it produces reports. For an engineer trying to enter their hours it's an hours long torture session every two weeks.
(Microsoft uses SAP as an ERP).
