Well you just let the cat out of the bag,so its not much of a secret.

Many players will already have created a full set of  some equipment. 

Multiples in fact lol. 

Exactly.

Though completing a desireable set gives sense of achievement, its not unusual.

Though the latest secret achievement may be an exception, most of the rest appear to be  a set of complicated linked actions,not typically performed, and perhaps intended originally like the rest to be gained accidentally or over time and not meant to be speed done.

Another problem is they have levels  and there doesn't seem to be much scope for levels when creating a set, which would be done as a matter of need anyway.