Because there are other countries who are delivering high-quality healthcare for less, and our (USA) healthcare expenditure is outpacing GDP growth.
There is no economic law that says the distribution of your expenses over various expenditures must stay equal. If you think about it, there can't be such a law; if it held, since nobody spent anything on electronics/flight tickets/Big Macs/etc. a century ago, you shouldn't spent a cent on them, either.
The richer we are, the less we need to spend on essentials such as food and shelter. That leaves a greater fraction of GDP for things such as healthcare. Most people will say they value health more than possessions, so what's strange about allocating more money to it?