Why is Uber worth almost as much as Boeing? 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.
A big part of the answer to this question must surely be network effects.

Yes, Ford has a lot of engineering talent. But so has GM. This means that Ford has to compete with GM to sell cars, and this competition limits that amount of profit that Ford can make. 

Ride sharing is a different story altogether. Suppose you write software that completely replicates the functionality of the technology used by Uber. How many drivers will want to use your software? Probably few or none because all of the people looking for a ride are using the Uber app and drivers want to be where the customers are. How many end customers will use your technology? Probably few or none because all of the drivers are using Uber and consumers want to use the platform where they can easily get a ride. As you say, people can easily open the new competing app to check it out, but if they find nobody else using it then they will just switch back to Uber. The only way to a competitor to overcome this challenge would be if many users simultaneously decided to switch at once, but such coordination is fairly rare in practice, and usually only happens if the new technology is vastly superior to the old one.

This property—that a product's value depends upon how many other people are using it—is known as a network effect, or network externality.

If you think about it, you will see that many products that have this network effect property are monopolised (or at least have relatively little competition). For example, 


a social network website is more useful if more of your friends are on there (a network effect)—and Facebook is a pseudo monopolist in this market. 
Developers write software for computer operating systems that have many users and users want to use the system with a lot of software available. These netwrok effects helped Microsoft to a position of dominance. 
Blu-Ray players are more useful if lots of shops sell blu-ray discs and shops will sell blu-ray discs if many people own a blu-ray player. The network effects here explain why the competing HD-DVD format died out and we were left with a single dominant high-definition media format.


There are many other such examples.

Since people understand that markets with network effects tend to "tip" to monopoly, and since monopolies tend to be extremely profitable, investors think that Uber is a good bet for future profitability.