Suppose, you have equipment to produce ASICs. Why would you want to sell those ASICs to anybody instead of mining bitcoins and other coins yourself?

You don't understand the whole context of the Avalon project. It is very simple:

Avalon was started by 2 young engineers who had almost no money (*), and no or little experience designing ASICs. They had no choice but to sell pre-orders at a low price (due to the overall risk and uncertainty of the project) to acquire the initial capital required to develop a 110nm chip and the machines ($200-300k or so). Once they successfully manufactured the first batch of Avalons, they honored the pre-orders because they are ethical persons. If they had say at this point "f*ck you, customers, we will mine with your hardware and not ship it to you", this would have enraged the community, destroyed their reputation (so no batch #2 and no batch #3 sales revenues for them), and it would have possibly disrupted confidence in Bitcoin due to 2 guys controlling the majority of the hashing power, possibly destroying the exchange rate and their potential mining profits.

That's why. If you knew the 2 guys, you would see they care deeply about the future of Bitcoin and about ethics.

(*) ngzhang did not make that much profits from previous FPGA sales, and yifu did not bring capital AFAICS.