Yeah also explain to me why someone pays 30 million dollar on an Rembrandt painting
Exact same thing as for tulips and Bitcoins: Pure speculation, zero value.
The point is that Bitcoin might have a value as it enable to transfer money over the internet.
In fact, it is already used (just take the Silk Road example). So it has a value. Probably very low but it still has. It is obvious that, the more people use bitcoin, the more the real value is increased. (If it is not obvious to you, you have to start with the basis).
Speculation is thinking that more people will use bitcoins in the future, hence increasing the value.
There are two kind of scenarios. Either bitcoins is not used at all anymore for whatever reason and thus become a pure speculation tool (like the tulip bulb).
Either it is mainly used as a speculation tool but still has some value. Example : gold. Gold is used in many technologies but its main application is speculation.
Either it becomes widely used and speculation is only done by a minority (example: fiat currency).
Your reasoning is that people only see the immediate use. But if it was the case, gold would be hundred times cheaper. The reason is simple : even if gold was completely useless, people trust that gold can be accepted anywhere against real money. It's only historical but this means that there are some trust in gold. People trust its value and thus accept it.
With bitcoin, we are exactly in the same situation: we are building that history.
Now, you can believe that we will fail. Right. Then sell your bitcoins if any.