you`d have to fit the complex with racks or shelves or something to setup the machines, hire a company to distribute and connect the electricity safely, hire a company to setup the network and also rent or buy expensive network hardware to support 1200 individual nodes, and hire IT and networking specialists to keep it running.
you need employees to set up the machines when they are ready, and a team to maintain it and trouble shoot once it is all up and running.
you have to train all new people every time your new product is reaching the production stage or keep them on as staff fulltime the whole year.
Best answer so far.
It obvious really why they sell. If you are a company that has manufactured lets say 1000 mining machines you have two choices in this scenario:
1) sell them for 1k USD each
2) mine with them to make 1.5kUSD >per machine< over 6 months.
With option 1 you have realised 1 Million USD within the time it takes to sell out - lets say 1week. So after 1week you have 1Mill in the bank and you can put half of that that money towards creating new hardware strightaway and keep the other 500k USD as profit.
With option 2 you have 1.5Mill but you get that progressively over 6months so you can't after 1week put 500k USD towards making new hardware. You will have to wait maybe 2months before you have 500k to spend on new hardware which is quite a delay over 1week. Also with option 2 you need to hire premises for the 1000 machines and you need to pay for electrical installation work at the new farm and pay staff to run it. After all that is paid for you could be waiting 3 months to generate 500k USD in profit. If your business intends to keep manufacturuing new hardware and chips this wait time is not feasible. So after rental costs for your farm (plus staffing, insurance, rates etc) your total earnings from 6months of mining the 1000 machines would be closer to the 1Mill you would have made from selling them 6months earlier.
If you expand that selling vrs mining scenario into a 5 year business plan you would quickly see how selling machines builds up a larger business over a shorter time so long as the business keeps reinvesting in new hardware production.
Remember when you sell the miner to a customer all the costs associated with keeping that miner on a farm (rent, staff insurance) are bourne by the customer too.
If you introduce the per-order side into this industry then option 1 becomes even more profitable.