I don't understand why people spend 1700 dollars on a mining rig
Bitcoin is not out of the woods. There are severe risks yet ... there could be a determination in the U.S. that Bitcoins are stored value used internationally and thus any business that accepts them as payment or for exchange must do full AML / KYC compliance, even for a 1 satoshi payment. (Of course, this is not at all likely to happen but is in the realm of possibilities). That would crush the bitcoin exchange rate for a while.
So if you had a GPU rig, you could always sell the hardware and get back maybe 75% or more (or more than 100% including the mined BTCs that had already been cashed out). Whereas is you bought BTCs, you take the entire loss.
That has changed though. Used GPUs are not selling near their "as new" price anymore. In a BTC/USD collapse, they'ld get even less. Used FPGAs may have value, depending on the model, but probably nowhere near the level used GPUs sell at, compared to "as new". Used ASICs end up essentially being paperweights at best.
So, historically, GPU rigs were nearly guaranteed to be profitable (in terms of USDs invested) no matter which way the exchange rate went.
Anyone buying one today either has free electricity or
doesn't understand math hasn't performed the proper profitability computations.