Sometimes, the obtuseness of people.
Depending on the width of the road and other factors such as walls, fences, kerbs, parked cars, gates, gate posts, pavement angles, visibility onto the road etc etc etc mean just reversing or driving straight off your drive is not always easy.
If someone parked either directly behind a drive or right up against the dropped kerb and any other factors mentioned above were involved, it might make it awkward and potentially hazardous to get off a driveway.
Again, the OP is not complaining about legally-parked cars or trying to ban them from parking. She is questioning why they choose to park on a road rather than a car park, which is a reasonable question. She is also questioning why, when she approached the parkers in a polite way, they have continued to park in a way which inconveniences residents.
For actual fuck's sake. MN fucking lives for parking threads, yet when one comes along, people get all pissy about it and blame the OP as though thus was some other forum entirely. Have none of you read an MN parking thread?? This is not how it's meant to be!