Herein lies the problem with self-soothing: the things that children find to distract themselves from their anxiety, and to allay their own anxiety, are very likely to be problematic behaviours. The more anxious the child becomes, the worse the behaviour will be (which is why isolating a child who is performing the behaviours in isolation is not going to stop the behaviour.)
From head banging, frantic rocking, nail biting, hair pulling (even eyelash pulling) to hair twisting, thumbsucking and more, the problem begins and ends with their psychological need to be soothed by others.
Toddlers do not self-soothe well because they are not mentally equipped to do so. They need the care of adults. Even small babies know, deep down, that if something bad happens, some animal attacks, or the adults leave or even just stop feeding and caring for them, they will die and there is nothing they can do to protect themselves. They know they are fragile and nearly completely helpless.
Everything that provokes a child's self-soothing behaviour is inappropriate for them to handle alone, including loneliness, fear, hunger, pain, and grief.
If human children were supposed to be able to manage all by themselves, we'd lay them and leave them --not consider childhood to be 20 years long.