Maybe because an ATCO may see the qualities required in future trainees better than HR?
Maybe because an ATCO can weedle out an applicants' motivation more than an HR 'person'.
God know's how motivation is being gauged now, if as marco888 says, they seem completely indifferent to visits to units!!
Take the time to read all of my post (and note the smilies) and you will realise that it was made a tad tongue in cheek.
Of course it makes sense for ATCO's to be involved in the recruitment of other ATCO's, it's just that NATS don't see it that way!!