There are a lot of such creatures - you can look at a kangaroo, for example, and rejoice at what an advanced person in this regard. Why helpless? Because it takes a long time for the child to load all the useful information into his head. In a child, the most developed part is the brain, and they need to be loaded for a long time and persistently, because we have practically nothing innate, almost all of our behavior is acquired. So we get it half of life, and then we use half of life. Live and learn.
For the more helpless, see marsupials, in a kangaroo at birth, the hind legs are generally in an embryonic state.
The duration of childhood and helpless childhood in humans is due to neoteny, that is, the preservation of children's traits at a later age. That is, if you compare a person and other great apes, the seams on the skull of a person ossify later, which means they give more time for brain development. And in general, a longer childhood means a longer period of study, and a longer period of study means the possibility of developing a culture.
First, it should be explained that there are breeding strategies. There are two of them - R and K. The R-strategy is to give birth to a maximum of cubs in a minimum of time. Representatives of the living world with a similar strategy are protozoa, plants, coelenterates, worms and some insects. That is, structurally not very complex types. The advantages of this strategy are that of a million cubs, only the best survive to reproductive age. It is species with such a strategy that give rise to ecological disasters - after all, if any important indicator (temperature for example) temporarily improves, then out of a million not 10 individuals will survive, but 10,000 ones. The consequences are sad. K-strategy is used by constructively complex organisms , including humans, and it lies in the fact that 1-30 helpless individuals are born in need of protection and guardianship. The helplessness of the cubs is explained precisely by the constructive (and for some, the behavioral) complexity of the mature individual - after all, a complex system must take a long time to form. But the selection factor is also great here. Mortality in 70% of cubs is the norm even in the world of highly developed mammals.
Regarding the second question, there are no more helpless cubs in nature than in humans. And this is also explained simply - a person is the most complex organism from existing ones.
As for whether there are more helpless ones, I don't know, but the reason for the helplessness of a newborn is a person's upright walking. Due to upright posture in the process of evolution, the pelvis has become narrower and in order for the child to pass through the birth canal, he is born somewhat prematurely compared to other species, and since a person does not have a bag, he has to "wear" on his hands. For the same reason, babies scream, this is the only defense mechanism available to them. If cubs, for example, some ungulates, can immediately after birth run away after adults to where it is safe, and there already lie quieter than water below the grass, then this is inaccessible to a person, therefore babies scream so that they are not forgotten, quickly picked up and preferably did not let go.
Grunts and carnivores, for example, are born completely blind. And if you compare with marsupials ...
In general, the point is whether this species of animals has a den. The hoofed animals have nowhere to go - they must immediately run after the mother. And if there is a den, you can walk less with a load in your stomach and protect the helpless child more