Because I see it all the time!
You apply for a job. At one stage you have to submit a CV, and then a covering letter.
Then at the next point you have to write recent work experience and qualifications- which is already on my CV.
Then you have to write a paragraph saying why you want the job and how you fit the person spec - which is already on the covering letter.
It the same with recruitment agencies. You go in to register, they’ve already got your CV, but you still have to spend half an hour writing it all down again by hand.
Why bother asking for all the same information twice??
AAAAAARGH