You could be right, however lighthouses are not modern inventions, the Great Lighthouse of Alexandria was one of the 7 Wonders of the Ancient World.
While lighthouses are ancient, they are not ancient in the U.S. The first lighthouse built in the U.S. was in 1716 according to the top Google results. Yes, Merlin could have built a lighthouse because they existed throughout his life, but why would he build one on Demonreach? The wizard's tower trope is confirmed as a thing by Harry, so while we don't know the reasons why wizards have towers, we know that they do have them.
@Morris: I'd say it's possible that's a possibility. Meaning, I'm not sure it would be a possibility under the time travel etc. rules in the Dresden Files. But why take special care with that one building even if it was possible?
The only reason we think of it as a lighthouse is that Harry just assumed it was a lighthouse when he didn't know anything about the island. I think we have more reason to assume it isn't a lighthouse, or at least not originally. I've also always wondered what happened to it and who built the cottage out of the fallen stones. Maybe that's why Rashid isn't "allowed" on the island?
A lot of people on various message boards have suggested various sources for the wizard's tower trope. One reason was that various astrologers, astronomers, alchemists, and a few other "wise men" used towers for practical reasons, and these wise men were associated with wizards. Another reason is that many wizards were in castles with many early castles just being a tower with a wall around it. The final common reason is basically that Tolkien did it and DnD ran with it.
The more Watsonian reasons that a wizard would have a tower included the reasons anyone would have a tower as well as magical reasons: they demonstrate power (it was a difficult feat physically and intellectually), defense, observation (of approaching threats and the stars), wizard's usually weren't associated with enough people to need a full castle, a tower is tied to both the elements of the earth and air, some suggested that magic gets grounded out near the Earth's surface, solitude, and finally that it was a good way of protecting the peasants from any spectacular failures by the wizard.
The metaphorical reasons included bringing the power of the heavens to the earthly realm, arrogance and hubris, and isolation or separation from normal people.