For the same reasons a TV isn't an investment.
You have to look at pianos as falling into the category of technology. They're continually changing instruments. You may not always be able to tell, but the piano industry is always finding better materials, better construction, better engineering, which all result in the pianos of today being far better than the pianos of 50 years ago.
Besides the continual change of technology, the piano market is flooded. Pianos are not a rare commodity, and a half-way decent instrument really isn't even all that much money anymore. It's great! But it means that your "vintage," "antique," or "rare," piano is not ever going to fetch the same price you, or your ancestors paid for it, let alone appreciate in value.
In piano language, "vintage" translates to "beat up," "antique" translates to "expired and too worn out to fix," and "rare," means "discontinued for good reason."
If you're in the piano market and are having a hard time justifying a large purchase that will never make you money, look at it as a different type of investment.
You are investing in the culture of music and the community it creates. You are investing in your emotional health and overall quality of life. You are investing in brain development and the stimulation that learning a new skill brings. You are investing your time into something other people can benefit from and enjoy.
Seems like a pretty solid investment strategy to me...
Oh, and you didn't think twice about buying that expensive smart TV, and that isn't an investment either...