Because the author wanted to build an indoor spin class bike, not a road bike on a trainer. Now, you're not wrong. When I was racing I'd take a bike very similar to my race bike (because when you race, you don't just have one bike), stick a trainer tire on the rear wheel, and the trainer had power and cadence readings built-in. The trainer also cost considerably more ten years ago than the author spent recently.

Maybe the author just has the one bike and doesn't want to unhook all the shite if there's a freak winter day when it is not rainy and cloudy in Seattle. Maybe the author is just a dorky geek that likes to build stuff. Maybe if I were still a hardcore cyclist, I too would try and see how much lipstick that $300 pig will hold. 'cuz I'll admit that my first thought was, "what, you didn't try to cobble a power meter onto that thing? Hold my beer..."