The main issues with enterprise systems is that their switching costs are really, really high, the return on investment for switching is much lower (unless the product itself is enough of a savings to justify the switching costs) and some regulators impose very specific requirements on them that make using alternatives willy-nilly difficult.

Also, these systems were modern when the company bought them and are often not maintained by the company itself (i.e. outsource SAP support to SAP consultants or Wipro). It is also common for a non-technical executive to authorize these kinds of purchases, though this is changing (which is great). For them, the brand matters a lot. ("Nobody's been fired for picking IBM.")

Also also, the consumers of these products don't really care about the dread involved in using them until the pain begins to affect their career directly. And when that happens, it will become ITs job to work around the dread while keeping the product instead of replacing it outright.

By the way, terrible internal tools are rife in every industry, including tech. Google has some really, really shitty looking internal tools for core production services. For example, their bug tracker (Buganizer, as of two years ago) was a HUGE pain in the ass to use compared to Github. I am surprised that nobody went ahead and Material Designed it as a promo project.