I’m guessing some other parameter was more important. Thermal expansion characteristics, ease of tooling, interactions with the materials around it, resistance to some other non-oxidative chemical corrosion…

This particular valve sees temperatures above 600°C. You basically need to use steel or inconel.

bearings and bushings need to be hard to resist wear and maintain their dimensions. Stainless steels hard enough for the job are too brittle and those other metals (Al, Ti) aren't even close to the hardness of any category of steel.

Stainless doesn't corrode, but it tends to corrode any other metal it touches. Depends on alloys and a bunch of other factors, but all stainless isn't the answer either.

The 737 Classic was introduced in 1984.

And the engine isn’t even made by Boeing.