Shipping and oil refining are different. If you apply capital to over come the cost barrier of entering those businesses, you can still access customers and potentially put all that capital to work generating profits.
With last-mile internet, if you lay in a redundant competitive network in a neighborhood - the capital efficiency of both networks goes down, with no mechanism to make that capital work more. You're limited to the houses in the neighborhood. So in that case you have high cost barriers, and natural monopoly conditions limiting how well that capital can be used to generate return...
If all the gas stations in an area are owned by Exxon & priced too high, the customer drives their car elsewhere. There is a very, very high capital barrier to entering the market, but customers still hold choice.
You can probably observe some region-locking effects short of monopoly by relate the cost of gas in come areas with ability to choose alternatives. e.g. Driving in the middle of nowhere, there are few gas stations and the cost of gas is high. Redundant stations aren't built in competition to one another along those stretches because the volume of traffic isn't high enough along the 'network' of the road, but within the range of the typical car, some will drive on and others will be forced the stop. So even that isn't as airtight a natural monopoly as last-mile internet.
With last-mile internet, if you lay in a redundant competitive network in a neighborhood - the capital efficiency of both networks goes down, with no mechanism to make that capital work more. You're limited to the houses in the neighborhood. So in that case you have high cost barriers, and natural monopoly conditions limiting how well that capital can be used to generate return...