Is the reason people buy them.
The initial idea behind them (bitcoin in particular) was for use as an alternative form of currency that could be easily and anonymously used to pay for purchases (like cash but not subject to Bank and govt controls that devalue it). However, very few people are buying them or using them for that purpose.
They are being purchased and used as a form of speculative gambling instead, and like many forms of volatile investment medium, they just don't stack up. Many better investment mediums are underwritten (or backed) by something tangible that has an inherent value (like property, gold, silver, most other commodities etc) that while subject to speculative whims, still maintains a base value that never reaches zero. That is not true of crypto currencies, they are potentially as worthless as any paper money that has been and gone before them.
Whilst arguments can be made for the benefit/need for appreciating values to cover mining costs and uptake growth, the absence of a value cap (such as might be achieved by linking the to gold or some other historically stable commodity) allows speculation to drive the price up exponentially, and what ever goes up must eventually come down. The fact is, that other than demand (the will of people to buy them) there is absolutely nothing to stop them crashing all the way down to zero (like a number of them have after their ICO).
They are being used as a basis for a pyramid game just like any other (that pretends to offer something tangible when it has none) and will finish badly for those that invest late.
No, I'm not a financial wizard or economist (who never get it right anyway). It's just good old fashioned common sense at work. If you want to gamble, fine, but at least know that that is what you are doing when you buy or invest in cryptocurrency.