It comes from spanish word which means "royal." El camino real means the royal road . Hence real estate means the royal state.
At the beginning of industrial age around 1500 power was no longer based on the land and agriculture. The monarchs realised they had to change in response to the land-reform acts that allowed peasants to own the land. So royalty created taxes on land ownership and mortgages, as a way of allowing commoners to finance their land.
So once royalty realised that money is not in lands but in taxes, monarchs set up banks to handle the increased business. Therefore today also land is called real estate because no matter how much you pay for it, it never really belongs to you. It still belongs to the royals.