Why Wizengamot needs witnesses?

In many contexts in Harry Potter series we see that even the most skillfully hidden truth can be unveiled by a skilled Legilimens. As in Kreacher's case where Dumbledore learns the truth about Sirius or in the case of Hockey, the house elf or Morfin Gaunt.In both these cases Dumbledore is able to extract out the truth.Then why does the Wizengamot simply not hire a accomplished Legilimens for conviction instead of depending on witnesses as in the case of a Barty Crouch junior or Harry himself when he appears for the hearing for performing underage magic.??