Because they shouldn't. If you want to speculate on BTC, just buy it. If the cost to mine each BTC today is higher than the cost to buy it today then mining doesn't make sense regardless of future price.

the real economics of buying Bitcoin mining rigs should be as follows:

If the cost of the mining unit will buy more BTC than it will generate, just buy the bitcoin. So a 2.2ghash miner (e.g. Blue/Red Fury from BPM) will generate 0.1BTC over 6 months. That miner would cost you approx $280. That would buy you 0.35BTC at a price of $800 per BTC.

There is obviously a point at which these would be economical to buy but that price point is less than $80, which is just over a quarter of their current price and that is probably close to what the manufacturer is paying for the production of them (including chip costs etc).

Source - I resell a lot of mining equipment on ebay, I charge approx 2 - 3 times what I pay for the units in bitcoin and i sell out of stock usually within 48-96 hours.

Now it may be that obtaining bitcoins is more difficult that obtaining mining equipment, or the fact that you can buy them on ebay (a site people are familiar with) as opposed to a site like localbitcoins (where people are not familiar with it and it requries sending money direct from your bank account) means people prefer to buy miners instead of BTC. The argument that coins will be worth more does not factor in as you can buy more coins now whoch will also be worth more.

Final point I would make on this is that mining has not been a profitable endeavour for most of bitcoins history. Anyone who mined from 2009 to late 2010/early 2011 will probably have done so at a loss, based ont he understanding that coins would be worth more later on down the road. The big difference however is that obtaining coins now is much easier than it was in 2009/10 if you are looking to obtain them without mining.