Series that run continuously replace their opening and ending animation every few months rather than keeping one for the show's life. The interval is contractual before it is creative.
Theme songs are commercial partnerships
Opening and ending themes are typically supplied by recording artists whose labels participate in financing the production and gain promotion from the placement.
Those arrangements cover a defined number of episodes. When the term ends, a new partner takes the slot and a new song arrives with it.
Rotation lets a production work with several artists across a run, which multiplies the promotional reach rather than concentrating it in one release.
The sequence is animated separately and reused
An opening is produced as a self-contained piece with a higher density of animation than a typical episode, then reused every week.
Amortizing that cost across many episodes is what makes the quality possible. A sequence used only a few times could not justify the same effort.
Changing it therefore has a real cost, which is why changes cluster at intervals rather than happening whenever the production feels like it.
The sequence tracks the story's state
Openings show the current cast, the current antagonist and the current setting. As a story advances, an old sequence begins to depict a situation that no longer exists.
Long-running series use the swap to signal a new arc, telling regular viewers that the story has entered a different phase before the episode begins.
Some productions play with this deliberately, withholding a character from the sequence or altering it slightly as events change, which viewers read closely.
Episode timing depends on the fixed slot
A broadcast episode occupies a fixed duration, and the opening and ending consume a known portion of it. Everything else is written to the remainder.
That is why episodes occasionally drop the opening entirely, using the recovered time for a story beat that would not otherwise fit.
Omission then becomes an event in itself, since a regular viewer notices the missing sequence immediately and reads it as a signal of significance.
Streaming complicated the arrangement
Music rights are often granted by territory, so a theme cleared for broadcast at home may not be cleared everywhere a series is streamed.
Services sometimes carry a substitute sequence in certain regions, or a version with different audio, which is why viewers in different countries can see different openings.
Skip controls added another wrinkle, since a sequence funded as promotion is bypassed by a substantial share of the audience it was meant to reach.