Things have value because we say so. How else can you explain the worth of a $100 banknote being higher than the paper and the ink it's made of? Bitcoin is the same.
All money is make-believe.
All money is make-believe.
You can also get into "everything is make believe" easily enough, so this isn't very helpful either way.
If you think of Dollars as "get out of US jail" cards it's pretty easy to see how they have intrinsic value.
The IRS not accepting cryptocurrencies is irrelevant; you can take a look at any trading platform and find that there are people, right now, willing to part with more than 49000 of their "government backed US dollars with their intrinsic value and ability to pay taxes" to acquire a single bitcoin.