The 3 & 4 stripes date back to the 1950’s when the airlines were in competition with ocean liners. They wanted to present an airliner as being the flying equivalent. These days even most commercial pilots may not even be aware of that.
Two stripes was the flight engineer / navigator back in the days when they had them. These days there is so little need of that its usually another non-captain ATP filling in. Consequently the two stripes rank now goes to the person in charge of the passenger cabin - a non pilot position. Anyone else on the flight crew could wear the uniform with one stripe if they wanted to, and I have on occasion seen male stewards do that.
But as has been said - these are costumes, not uniforms. You could wear admirals rank on your pilot shirt if you wanted to.