Why roach control usually falls on the landlord
In most cases, yes. A California landlord is generally responsible for treating a cockroach infestation, especially when it is building-wide or comes from conditions the landlord controls, like gaps in the structure, moisture, or plumbing.
Keeping a rental free of vermin is part of the landlord's duty to provide a livable home. Because roaches so often spread between units and hide inside the building itself, an infestation is usually the landlord's to handle with professional pest control — not something a tenant is expected to fix alone with store-bought products.
Responsibility can shift if a tenant's own serious neglect clearly caused the problem, but that is the exception rather than the rule. Most apartment roach problems trace back to the building. If your landlord is refusing to properly treat an infestation, it may be worth getting guidance about what the law requires.
This is general information about California tenant rights, not legal advice. Every situation is different.