They evolved in a harsh arid environment, in this case Madagascar is where the tree in the picture is from, it's endangered too. Trees evolved way before baobab did this, the trunk swells up with water to survive the desert environment.

From the nature paper, they seem to be quite young, around 41million years ago, and the current species Madagascar around 20+myo.

Generally nature doesn’t keep doing a useless thing if there’s no longer any need to do it. Energy efficiency is a constant selective pressure in the absence of all other challenges.

My bet is that baobabs are shaped that way for very good reasons. The fact that the trees are spaced far apart even in baobab forests is a clue: the environment is very harsh, especially on saplings.

Since baobabs reproduce via many fruits and since they can be spaced very far apart my hypothesis is that they evolved to be very tall with featureless trunks in order to attract fruit-eating birds to carry their seeds. The tall and featureless trunks would make the trees difficult for ground-dwelling predators to climb, keeping the birds and their nests safe from attack.

I believe leopards are fairly common in these areas and they love to climb trees, although they prefer ones with lower, wider branches they rest on and even eat their prey within. Leopards have been known to carry large prey such as gazelles up into the branches of a tree to protect their kill from being stolen.