Because they can't regulate their own temperature.
At what age does that stop?
My 23 month old was just in the garden with a long sleeved t shirt and cardigan on. He went to take the cardigan off but dp told him to leave it on, I told him it was fine, it's not that cold out here.
Then dp mentioned the one extra layer 'rule'. I said I'm not sure it counts at this age. If he's old enough to take his own jumper off then he must know he's too warm with it on!
So can you clear this up for us?