Bitcoins are traded on the exchange, they invest money in them.
Since 2015, bitcoin has risen in price from 200-300 dollars to 4000 and continues to grow. This made the crypto currency interesting for investment and speculation.
Interest is fueled by success stories. In 2009, a Norwegian student Christopher Koch at random bought bitcoins for $ 24. He remembered about investing in 4 years, when his fortune reached 885 thousand dollars. And I bought an apartment in Oslo for a portion of the invested money.