It's scale.
There are tens of millions of boaters and more than 250,000 new boats sold each year, compared to 700,000 pilots and a few thousand planes. It's literally two orders of magnitude difference.
Think of it this way: Assume it costs $20M to get a new plane or boat designed, prototyped, tested, certified, and released to the market (completely fictitious number, but it makes the math easy).
Now, let's say that you can sell 500 new planes over 10 years vs 50,000 new boats. That's $40,000 in capitalization cost per plane vs $400 per boat.