They did. Ancient Rome and contemporary Japan are two examples relying heavily on adoption to pass on property to the next generation. The biological reality of genitors is only partly relevant to determine family ties.

Because often the point was to ensure a pooling of the resources of two families.

But adoptions to secure an inheritance chain is historically certainly not that unusual either.

In its historical roots, though often (but far from always) without the "orphan" part, that's exactly what marriage is: the process by which a family chooses some non-family person to share with some member of the family in the inheritance of the family. (Sometimes in a reciprocal, but even when so often not symmetrical, relationship with another family.)