Because the certification and operational rules require accounting for the different configurations as the takeoff progresses, and each such configuration/portion of the takeoff has its own rules, generally, so the takeoff is considered to occur in segments.
Similar techniques are used for the landing data collection and generation (especially after Douglas showed what can happen when you try to do a continuous landing for performance data) - although the term landing segment isn't used, it certainly could be, if consistency were required.
§ 25.111 Takeoff path lays out the segment concept in subpara (d) (my emphasis)
(d) The takeoff path must be determined by a continuous demonstrated takeoff or by synthesis from segments. If the takeoff path is determined by the segmental method—
(1) The segments must be clearly defined and must be related to the distinct changes in the configuration, power or thrust, and speed;
(2) The weight of the airplane, the configuration, and the power or thrust must be constant throughout each segment and must correspond to the most critical condition prevailing in the segment;
(3) The flight path must be based on the airplane's performance without ground effect; and
(4) The takeoff path data must be checked by continuous demonstrated takeoffs up to the point at which the airplane is out of ground effect and its speed is stabilized, to ensure that the path is conservative relative to the continous path.
The airplane is considered to be out of the ground effect when it reaches a height equal to its wing span.