Why do the creators of ASICs not just keep them for themselves?
I can't really answer this without speculation, since I'm not an ASIC developer myself. Please keep that in mind when you're reading this!

There are a two paths that ASIC miner manufacturers have to make a profit.


Expend fiat to produce mining equipment

Use mining equipment to mine bitcoin

Sell bitcoin for profit

Profit


Accept fiat or bitcoin in exchange for ASIC mining equipment

Profit




The shortest path to profit is to sell the device. Existing bitcoin is worth more than non-existing bitcoin right now, especially when that existing bitcoin can be turned into more mining equipment.

Keep in mind that they can also do some of both. While it may not actually be so, it's fair to believe that employees of an ASIC manufacturer are likely to invest in their own product by purchasing one.