Why don't some people serve from directly behind the line but sometimes one or more meters behind it?

Unless you can start the serve from twice the height of the net, you are relying on gravity to pull the ball down into the court. The harder the serve, the straighter the ball.

For a given height at which you can serve from the back line, there will come a maximum service speed at which the ball cannot both go over the net and land in the court.

By serving from farther back, you can have the same forward speed on the ball, but since it has more time in flight, the vertical speed as it crosses the net can be higher, allowing the serve to land in the court.