How to build a Bitcoin mining rig?
The people who got rich during the California and Alaskan gold rushes were not the gold miners, but the business who sold the shovels (and other supplies). Or at least that's how common wisdom goes.
And it makes sense. Anyone can buy a shovel or a bitcoin mining rig, there's not much barrier to entry. Profitability should approach break-even or worse as new entrants flood the market. But only a few businesses will dominate the market for bitcoin mining rigs, which requires business acumen, manufacturing contacts and a good brand (marketing), etc.
There is another reason, of course: CASH FLOW.
If you are relying on "pre-sales", you're building a business using your customer's cash. That's awesome from a startup perspective. And you don't want to be competing with your customers! (So if they are keeping the best equipment for themselves they'd best keep that quiet.)
If they instead built equipment for their own mining, they'd need all the investment capital upfront to finance it. It would also be a much riskier proposition given the variance in bitcoin price (especially when you try to sell it in volume) and the minimum orders for ASIC hardware manufacturing. You could say more reward potential sure, but not without corresponding increase in risk. By selling the machines they are transferring much of the risk to their customers and reducing their variance. It's unlikely their manufacturers will take bitcoin in payment.