The high cost of access could well be the point : if you can easily hire a boat to get to your private island, it's pretty simple for governments or peoples to do the same, club you, and take your stuff. A hundred thousand bucks would cover invading you, and make good return on investment.

By contrast, you'd have to have something of very high value to cover a rocket launch, and that something must be mobile enough to send down easily. (Or in extreme cases, you might be the only people who retain full knowledge of the manufacturing necessary to make the rockets, in some way that isn't easy to reverse engineer -- see the difficulty we have reproducing several engine designs as a guide here.)