Why does the bone grows up into the length and not into the width?

Bone growth is at the end of the bone, not the middle. It actually pushes up the very end right at the “cap” of the bone. Like this. <=====>

Where the arrows are, that is where the bone grows. The rest of it ossifies, although the marrow keeps the interio healthy.

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Presumably the vertebrate species whose bones just grow aimlessly in all three dimensions were not as successful in reproducing as those of us whose bones grow according to a genetic blueprint that enables food-gathering, defense and reproductive capabilities.

Genes are incredible instruction packets that tell each type of cell what to do. From the time of conception, the cells begin to divide and they eventually become more and more specialised. By the time our bones are in place, the instructions in the genes in the cells’ nucleus is telling the bones how fast to grow and where to grow.

Here is a great website. I’m not sure of your language, but this is all in English. http://www.hhmi.org/biointeractive/biology-cells

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How come cells are so intelligent but people are not?

I think some people are very intelligent. Cells don’t always work as they should. Birth defects, cancer, auto-immune disorders…. not perfect. Everythings works just as well as it needs to for the perpetuation of the species. Nothing needs to be perfect. Just ‘good enough’.

I believe there are “zones” of growth near the ends of the bone, for example, thighbones. My guess is that expansion in the outward directions is resisted by the strength of the non-growing parts of the bone, but expansion along its length is not resisted because the non-growing parts are free to move away from each other.