Why don’t parents actually play and engage with kids?

When Adults play with toddlers, preschoolers, or lower level elementary school aged children, why don’t they actually play?

What is challenging for your child in a rock climbing race on the playground with you if they know that you are going to let them win, and that you are not even going to try to climb or you are going to pretend and climb incredibly slow?

Where is the challenge when playing monkey in the middle with your parents if you know that they are just going to give you the ball, if them as the thrower, are just going to deliberately drops/fumble the ball, or that when they are the monkey, they are just going to pretend to try and get the ball but then act like they weren’t able to, and just let you win?

What is challenging or fun in wrestling if the opponent in wrestling is just going to let them win and not even try, or just pretend during the whole thing?

I mean, if two parents or you and a friend were to play a game they would actually play, but then as soon as a kid is playing with them they deliberately let them win with seemingly no effort, they do not even let the child earn a win, or even let them show the adult what skill they have. The child could be in preschool, kindergarten, or they could even be a first, second, or third grader. The child could even be a next-door neighbor who you have just met regardless, the adult assumes that the child has no skill and therefore cannot play with them. For that child who was playing, where was the challenge?

What exactly would happen if the adults were to actually play an example that I gave instead of just pretending and quite literally just giving the kid the win even though the kid is putting no effort into it? What would happen if the parents actually played monkey in the middle, actually played with their child on the kitty Rockwall race, actually played during the relay race, etc.