Because we canonically summon a demon/aberration from a grimoire ritual, and then enter into a contract with it (for all the rational demons) or forcibly subjugate it (for the felpuppy because it is a demon dog and void aberrations because they require being subjugated as otherwise they’d consume reality).
While for gameplay reasons we can cha cha slide over to Demon/Aberration Renaming NPC and just “randomly select a new demon”, canonically this is not the material reality.
Canonically each time we are “changing our demons”, we actually would have to do the ritual again and enter in a contract with a new demon (or subjugate them for the 3 minions aforementioned) and release the previous demon (or aberration) from contract.
Unless of course warlock stables are added, in which case you could just endlessly keep any demon in a giant grimoire and just pick your favorites any given day.
All of which you would know if you read my warlock stables post 