In Japan it is quite common for an unrelated executive to be chosen as the successor, and they will then either marry a daughter of the CEO or even be adopted by him before taking over. This ensures that the company remains a “family business” (a desirable label in Japan) but it isn’t necessarily by blood, rather they become family via their success in the business.

So it’s often not nepotism as we know it - it’s almost reverse nepotism!

Edit: This is not the case for the Toyodas, however. All leaders of Toyota were indeed descended from the founder.

Note that There's more than natural selection at play here. I wouldn't be surprised if the offspring have been groomed from a very early age to take over the company. Is it such a surprise that they would seem best fit?