The dates on which clocks change are not a natural or astronomical fact. They are statutory, and the authority to set them is divided in a way that produces the patchwork actually observed.
The rule is written in law
National legislation specifies the start and end dates and the hour at which the change occurs, and those provisions have been amended repeatedly over the last century.
Because the dates are legislated rather than derived, they can be moved, and shifts of a few weeks in either direction have happened more than once.
Software carries the rules in a time zone database that must be updated whenever a jurisdiction changes its law, which is why devices occasionally show the wrong hour after a change.
Opting out is permitted, opting in is not
In the United States a state may decline to observe the change and remain on standard time throughout the year, and a small number do exactly that.
Moving permanently to the shifted time is a different matter, because that would place the state outside the national standard and requires a change in federal law.
Many legislatures have passed measures expressing a preference for permanent summer time contingent on that federal change, which is why such laws exist without taking effect.
Territories and neighbours diverge
Overseas territories and some tribal jurisdictions follow their own practice, and the result is boundaries where the local time difference changes twice a year.
Internationally the dates differ as well, since regions transition on different weekends, so the gap between two cities is not constant across the year.
Scheduling across borders therefore has to account for periods of a few weeks in which the usual offset does not apply.
The stated justifications have shifted
The measure has been argued at different times on grounds of energy use, evening daylight for commerce and recreation, and road safety.
Evidence on energy has been contested for decades, and the argument has moved steadily towards how people use daylight after work rather than towards consumption.
Opposition has grown around the disruption of the transition itself, which is why recent proposals tend to focus on abolishing the change rather than on adjusting its dates.
The transition creates real operational problems
Systems that assume every day contains the same number of hours misbehave twice a year, once when an hour does not exist and once when an hour occurs twice.
Transport timetables, shift rotas and scheduled processes all need explicit handling for those two nights, and the handling is a recurring source of faults.
These costs are borne quietly by operators rather than by the public, which is part of why they feature little in the political argument about whether to keep the practice.