Because, sometimes, partners of equal skill level are not available.
In addition, there is also ( in the US ) an age problem. many older ladies cannot find partners who are willing to dance in the comp. arena.
Proo/Am has " driven " the dance profession for over 60yrs.. it has, thankfully, produced some very fine dancers, Pro and Amat .
Would that have happened without that system?.. moot point .
Sometimes in the world of dance, we make do with the situation at hand, it may not be the most ideal, but it serves a purpose . And, by the way, that system is being promoted by the WDC .