Its embryological and that's just the way it is. Humans are stuck with brachial plexus for the foreseeable future. I don't see a thoracic plexus working. If we'd had a thoracic plexus the upper limbs could have evolved say on our trunk and we perhaps would never evolved as humans. If we'd missed out on an opposeable thumb we could still be living in the trees?

Plexus means network of nerves in the body.

There is a need of brachial plexus as the cervico-axillary canal is very narrow and many important structures pass here.

If there were no plexus formation here then all the branches would have to travel in this narrow space leading to traffic jam.

Similarly the lumbosacral plexus as it gives branches to supply thigh muscles and skin of lower abdomen, external genitalia and thigh.

We have segmental arrangement of nerve distribution along the thorax similar to earthworm segmental pattern.