- EDIT : Sorry for my bad english - First of all, it's not "that" easy. For people like us used to computers&such, yes maybe, but it's not for everyone (setting up the hardware, the software, managing wallets, securing them, placing orders on exchange plate forms, etc). Then, in some country the power price is high, so it's hard to generate a lot of profits. If you built multiple big rigs, heat, noise are factors too.
If all that is ok, then you need the money to start (buy the cards, mobo, special cases, riser, etc). The catch is, yeah Eth, Zcash, etc, are high right now, but it won't last, so making big investment is always risky. If you mine multiple crypto moneys, then, it can be very time consuming to study the market, sell when it's needed, etc, like any exchange stuff. I've a job, I only mine with my gaming card(s), I don't want to heat up all my apartment and spending all my time in that activity... When I read some people who do that at a medium or large scale, it can be a full time job. And a risky one.
At last, in some countries, the IRS (or FISC in France, anyway, all the "eh, we need a chunk of your money" agencies) can taxe your gain too... And frauding them is quite the sport...
I'm missing a lot of things no doubt. But, when you dig a little bit, you can see that it's not that simple, and it's not risk-free (plus, chances are it won't last, even if there is always a new crypto money, maybe your current hardware won't be good at it...)
IMO, a lot of people who buying a lot of stuff right now just because it's making the news will have a bad surprise in a few months if they haven't study this domain a little...