1) The people who pay generally do not use the system. This is true for enterprise software in general and leads to vendors prioritizing having all features organizations ask for (regardless if they are a good idea or not) and also prioritizing features management deems important over fundamental workflow, UX and polish in general.
2) EHRs are very large and complex and can almost always gain more customers by gaining even more features and replacing smaller more specialized systems. A typical EHR will have features for ordering tests and viewing results (for clinical chmistry, microbiology, radiology and more special stuff like physiology etc), appointments and resource planning (rooms, equipment, personnel, staffing), clinical notes including computing scores and values based on other values, medication (ordering, administering, sending prescriptions electronically) and administration (admissions, discharge, payment, waiting lists). That is a lot of different stuff!
3) Once a vendor wins a contract and installs their EHR, very little can be gained by improving the lives of users. Contracts and sales cycles are very long, and the vendor gains very little financially by improving the system. So many vendors are focused on charging money for customer specific features or adding new features to win new tenders.
I'm not sure what the solution is, public alternatives have failed spectacularly since they are typically run by public administrators who have even less of a clue how to develop software and what users want than the vendors.