That is not always the case. One can earn and do business with the money of other people as well. If you look at the businesses built around Bitcoin like the BTM, you actually earn by the Bitcoin of other people when they make the transaction. They are actually charged a certain fee while marking the transaction.

Besides, there are some bounty campaigns and participating in such campaigns incur you money which could be invested in any business which does not go form your own savings. Alibaba enterprise also works on the same principal and does their business without holding any inventory.