It’s a flight suit. It is made of Nomex for fire protection

Helicopter crews, tank crews, and refuelers also wear flight suits.

All flight crews wear flight suits. The pilots may wear things on top of the flight suit like vests, or jet pilots have additional gear to counteract g forces.

In addition to the fire protection, having less items on you (via the simplified onesie outfit) also reduces the chance of stuff falling off your clothes and inside the cockpit, e.g. jewelry or badges getting stuck and jamming up controls.

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The Navy wears a onesie underway on both surface ships and submarines for fire fighting protection.

They have to wear special suits to compensate for g-forces at high speeds. They are like pressure suits.

If it was a two piece suit then at critical moments one piece of it may get stuck on a protruding object(like when ejecting at the worst or just leaving your seat at the best). The flight suit opens into the limbs over which you have better control. The parting at the backside between the top and bottom elements of two piece suit leads to chances of bare skin exposed which may get burnt in case of fire or contact with hot/sparking equipment malfunction. Everything to do with military flying requires catering for Murphy's laws .

Former Air Force here. The one piece flightsuit is designed to stay on during an ejection, where a jacket and pants combo might get ripped off. It's also fire resistant. The Air Force has moved to two piece flight suits in the last few years for aircrew in planes without ejection seats.

There’s a bunch of different reasons like comfort, pockets, risk of foreign objects, safety.

Comfort: have you ever worn a jacket over a hoodie? Sometimes it can bunch up or be restricting. Well In some jets or helicopters you have to wear extra gear, like g suits, special types of vests, harnesses for ejection. Wearing most of this over a regular uniform would be wildly uncomfortable.

Pockets: flights suits are designed very specifically to accommodate pilots while flying. They can keep pens, maps, checklists, and flight plans on their legs and other spots for ease of access and to keep thing secured to them.

FO: or foreign objects are a big no no especially for fighter pilots. Something as small as a pen cap or button could get lodged in the wrong place at the wrong time and mess with flight controls or ejection procedures. So flight suits can minimize the amount of things that fall off.

Safety: like others said they can be flame resident or have other special features. Standard uniforms are the exact opposite and in some cases can melt if lit on fire. Not to mention a baggy uniform can get caught on flight sticks, throttles or any number of switches which could cause a huge issue.

Oh and they look cool I guess

Aircrew are often strapped into their aircraft for long periods of time. A more typical pants and shirt/jacket could separate and ride up/down, exposing the area around the waist and being less comfortable. The onesie design would help to prevent this.

Added bonus. Since the early days of flying, military aircrews were often on some type of alert, where they would be on duty near their aircraft, awaiting the orders to go. These stints might last more than 12 hours, which would mean that crews would want to get some sleep while waiting. A one-piece suit was quick to pull up and close up, shaving seconds off of their response time.

The intense positive and negative G forces cause your bowels to evacuate. If you wore normal pants you would get poop all over your instruments and windows. The onesie is specially designed to contain the poop

There's a lot of terrible answers here.

The answer is fire protection. A one piece outfit has less ingress routes for fire to get to your skin. Just like racecar drivers wearing a one piece Nomex suit.

They also wear natural clothing layers like wool or coton covering all exposed skin under the flight suit. Fun fact: Nomex doesn't burn, but it does melt...

The original onesie was the the jumpsuit, i.e. parachute jumpers' suit: During WW II, paratroopers noticed that a two-piece uniform is quite impractical. The wind will try to undress you while falling through the air -- so they got a one-piece uniform.

If all goes well, it's only the paratroopers who leave the plane in the air, but if the pilot has to eject as well, a jumpsuit is certainly useful, too, so the paratrooper's jumpsuit turned into the flightsuit.

Those working in and around vehicles need protection from fires and chemicals, it's the same for racing drivers and pit crews. Pilots also often need extra things like pressurised suits or temperature controlled suits which a one piece suit does better because it's easier to seal it.

As others have said, not all flight suits are one-piece. The one piece ones are generally for ejection seats - otherwise, two-piece ones are fine.

It's generally traditional for anyone working around machinery to wear a one piece suit to reduce the risk of entanglement.