An adult enters therapy and talks. A six-year-old can't do that — not because they don't want to, but because the ability to name an internal state and place it in context develops late. Asked "how do you feel?", they'll answer "fine", because that's the answer they've learned.
Play is the language they already have. A child who arranges figures in a doll's house and puts the father in a different room is saying something very precise. A child who endlessly repeats a car crash with toy cars is processing an event, not playing at random.
Play therapy isn't "supervised playtime". It's a structured method in which the therapist tracks recurring themes, watches what changes over time, and intervenes inside the play — not outside it, with explanations the child can't use.