United went through a period in the 90's where it was majority-owned by its employees, which ended when it restructured under bankruptcy in the mid-2000's: https://en.wikipedia.org/wiki/UAL_Corporation#History

In general, airline employees enjoy very strong unions, not only for pilots but also flight attendants.

Airlines are literally among the most capital intensive industry out there. Where are some pilots getting their first billion for aircraft, flight scheduling, landing slots, fuel, maintenance contracts, etc...

Notice how most employee co-ops such as law firms have essentially zero capex requirements. They don't need startup capital that necessitates shareholders.