At $68 billion, Uber has a higher valuation than GM, Ford, and Honda.
According to Microaxis, Boeing is currently valued at around $87 billion.
Why is Uber worth so much? I would think that judging the value of companies would be similar to judging the value of employees. People often make the case that your wage/salary should reflect how easily someone else could do your job. (See this question for example on why celebrities are worth so much). Along those lines, it is hard for me to imagine how anyone could replicate the engineering and infrastructure of a behemoth like Boeing, in order to provide equivalent transportation or defense capabilities without taking many years, many talented employees, and billions of dollars to get up and running.
However, I could imagine 1 engineer fresh out of college making an app identical to Uber in only a year. Additionally, there would be no cost for the consumer to download that app, and try using it (there may not be many drivers, but there's also not a large opportunity cost to opening the app, checking for drivers, and closing the app if there are none).
So how could Uber be valued so highly? If it disappeared tomorrow, it could be easily replaced quickly without much start up time, and it could be adopted just as easily by drivers and users (the very same ones working for Uber). If Boeing disappeared tomorrow, everyone would be in trouble.