Many (most?) democratic countries impose strict limits on how long you can be held without being charged. In the US, for example, you can only be held for 72 hours — at which point the police must either charge you or release you.
Even in situations where you could be held in jail, there's a tendency to let you go unless it is impossible to prevent you from jeopardizing the investigations by any other means. For instance, if the only real worry is that you flee to another country, you might have your passport confiscated rather than being held in jail. Likewise, if the worry is that you can use your influence to make others do stuff for you (stuff that jeopardizes the ongoing investigations, I mean), then you might remain at home, under surveillance, and so on.