It’s by design.
Same reason they’ll send someone from Stafford to Wolverhampton for a disability allowance assessment, if you can get to Wolves you clearly aren’t disabled right?
The assessments are often outsourced which is one reason it might require travelling. Also, who on earth thinks someone who is disabled is not disabled if they can travel? There's plenty of disabled people in the workplace travelling to work every day, they just often need their disability status for other reasons, Motability etc.