Freehand lines bow because your arm pivots around joints, so the natural path of your hand is an arc rather than a line, and drawing straight means either using a larger joint with a longer radius or moving the paper instead of the hand.
Your arm is a set of hinges. Hinges draw curves.
Ask anyone to draw a long straight line without a ruler and watch what happens. The line starts fine, then bows, usually toward the body.
It is not shakiness. A shaky line is rough but can still be straight overall. This is a smooth, confident curve, which means the error is structural.
Every joint in your arm is a rotation. The wrist pivots, the elbow pivots, the shoulder pivots. Rotation produces arcs.
To move your hand along a genuinely straight path, several joints have to change angle simultaneously, each at a continuously varying rate that cancels the curvature of the others. That is a real coordination problem, and your motor system solves it approximately.
The approximation is an arc whose radius matches whichever joint is doing most of the work.
This is the practical part.
Wrist. Radius of a few centimetres. Excellent for short strokes, hopeless past about five centimetres, because the arc curves sharply.
Elbow. Radius of roughly thirty centimetres. Good for medium strokes. On a long line the bow is visible.
Shoulder. Radius approaching seventy centimetres. The arc is so gentle that over twenty centimetres it is nearly indistinguishable from straight.
Which is the whole content of the advice draughtsmen give: draw from the shoulder, not the wrist. It is not about strength or steadiness. It is about picking the hinge with the longest radius, so the arc you inevitably draw is the flattest one available.
Lock the wrist, keep the elbow relatively fixed, and move the whole arm. The line gets straighter immediately, and it feels worse while you do it, because the movement is less precise and more committing.
Slow lines are less straight than fast ones, which surprises people.
Drawn slowly, you are making continuous corrections, and each correction is a small direction change. The result wanders. Drawn quickly, the stroke is closer to a single ballistic movement with one plan, and its errors are smooth rather than jittery.
Artists rehearse this by hovering above the paper along the intended path several times, then committing in one motion. The rehearsal loads the movement; the commitment executes it without mid-flight corrections.
Rotate the work, not the hand. If a line runs at an awkward angle, turn the paper so the line matches your natural sweep. Every joint has a direction it prefers, and fighting that is unnecessary.
Anchor and pull. Plant a finger of your other hand and draw toward or away from it. A fixed reference at one end of the stroke does more for straightness than any amount of concentration.
The interesting thing is that straightness is often not the actual goal.
Cutting something in half well is a question of where the line lands, not how straight it is. A slightly bowed cut placed at the correct point beats a perfectly straight cut placed two centimetres off. One is a small error in shape; the other is a large error in the thing you were asked to do.
Which is worth remembering when a cut looks clean and scores badly. Neatness and accuracy are separate skills, and only one of them gets weighed.