This is because the value of bitcoin still depends on fiat. In order for most people to actually be able to spend the bitcoin or any other coin they earned, it must be converted to fiat. Although, one may argue that there are now some stores and establishments that accept bitcoin, so why does it still need to be converted? First, the establishments that actually offer btc payment are still minimal. And second, how do you think do they earn money by letting people pay thru btc? Of course, they still convert it to fiat. Truth is, bitcoin would not really be popular to many, and only a few will be interested in it if it could not be converted to fiat. A lot of people are investing in it to gain profit, and that profit could be limitedly spent if it will not be converted to fiat.