Decades of legacy code to maintain continuity of business (dont assume for a minute you can export or import data from many of these apps in any sort of sane way). I think a lot of businesses would be open to better designed products, but the aforementioned legacy compatibility and the need to train people on the new software make this a nonstarter. Pretty much the same reasons Microsoft Office is still so dominant, although I actually like the recent versions of office.

My guess would be that everybody is just happy they have some tool for this other than pen and paper. Since not one product in the sector distinguishes itself by being both mostly correct and somewhat useable, there is no pressure to improve.

EDIT: typo

The sale for this kind of software is made at a level that has no idea what the actual product/deliverable does. With predictable results.