Because:
- miners are not servers and are actually built to be air-cooled, at least current designs
- oil is costly to buy, air is free
- you need special receptacles to buy, air is free
- you need heavy duty pumps, fans are cheaper
- you need to be able to retool quickly, miners come in all shapes,
- miners get obsolete quick and they might be thrown out in 1 year, unlike servers
- evaporative cooling can actually lower ambient temperature
- oil cooling STILL needs a fan to cool the heat
- redundancy costs and failure risks...