Surely the police who turned up to investigate would have reported back that it was just a scarecrow instead?. I can't imagine them repeatedly investigating the same scarecrow unless there was poor communication in that force.
Possibly, but with little detail, we don't know.
You are also assuming that the police knew people were misidentifying a scarecrow.
Here is my idea of why the police might not say "oh, it's a scarecrow". Again it's all based on assumptions people make:
Person in distress in a field is reported. Police go looking for a person in a field. Most people would expect/assume that the person is mobile and may not be found in exact area as originally reported. Even people reported lost when hiking are difficult to find because they don't stay in one place.
Truckers are also seeing the person while they are in motion, so exact location reported may be off. They are probably not familiar with the area ie local.
We don't know if several reports were close in time, only reports covering 20 years. If it were several within the same month, then perhaps the police might say there's a scarecrow over there. Maybe that's what they've seen.
A span of years would be difficult to recognize any changes in a field.
The police are also under the assumption that it is a lovers' lane and frequented by people so they are expecting that the people reporting have seen a person.
I wouldn't say the police were negligent or poor reporters, only that to compare notes spanning a large amount of time, and, probably no further discussion with the truckers, details can be missed.
Biases and expectations always affect what you believe happened.