As with every company that maintains 51% private ownership, they have value because in the future someone might buy them from you for more than what you paid for them.
Or, to put it another way, they have value because people will pay for them, and people will pay for them because they have value. Isn't finance ridiculous?
Well. Some people, like hedge funds or hypothetical corporate-takeovers, will always be interested in paying for them because it's a way to turn money (sitting around) into more money (in the form of a stream of income) - whether through a full-on acquisition, stock buybacks, or due to dividends. Assuming they get a good enough price, naturally.
Other people will pay for them because the first group will pay for them. That's not ridiculous, that's just a transitive property of things being worth money. It's only a problem and a bubble insofar as this group's enthusiasm overwhelms the first group's.
Or, to put it another way, they have value because people will pay for them, and people will pay for them because they have value. Isn't finance ridiculous?