because they don't know nothing about exact time of arrival of their ASIC and if they don't know about that, they cannot determine whether they can get full return of investment or not in reasonable time.
ROI per any given period will decrease every new ASICs hit the network, so if they get their ASIC after massive BFL's 10k ASIC chips hit the network, they will get 1~2 BTC/day not 5BTC/d like today.

and mining rate is not the only factor. USD/BTC rate can be fall drastically. even if you can maintain average 2 BTC/d rate for the next whole year, your 730 BTC can be worth much less than $20k.
730 BTC worth more than $90k today, but if exchange rate falls to $10/BTC again, it will be $7.3k. and $10/BTC is not even close to the basement. no one knows where the basement is for sure. could be $0.1/BTC.

Therefore, $20k price tag is perfectly reasonable for those who are in the waiting line, and not a bad deal even for a current holder of ASIC.
$20,000 price tag is actually 75BTC + premium. if you consider the premium is 50%, $20k for ASIC deal is actually selling 112.5BTC at the rate of $178/BTC.
If someone believe market will experience at least ten fold crash very soon, selling out BTC mining machine at the rate of $178/BTC is good deal.