It is not because of focal length, but magnification. The same subject at the same size will have the same depth of field, yet, to obtain that,  with a wide-angle you'd have to come very close to it and with a tele you would have to stay far away from it.
In practical life situations, it's the distance from the subject that matters. A tele flattens, a wide-angle creates unnaturally depth. Both deform. Because a tele magnifies, you get a larger background compared with the foreground, and thus an increased effect of depth of field. A wide-angle opens up/ makes the background smaller, hence more sharpness.

Which is why a wide-angle is so great when you have something very close and still can have a sharp background.

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