This is precisely the reason: a doctor’s interpretation as well as the liability that comes with it.
Good luck on reading an EKG graph yourself. It is apparently not even easy for a trained doctor to discern what exactly is going on.
Source: I worked on a long term ECG holter monitor sold by GE.
You're also paying for interpretation of your EKG at the same time. There are plenty of places happy to sell you home BP/HR/EKG monitoring kits, but you have to remember that you are paying for a doctor's time and clinical expertise when getting them in a hospital. It's not like a single EKG will tell you all that much about your heart anyway.