You can, and on a front tooth with a small access cavity it is sometimes reasonable. On a molar or premolar it is how people lose teeth that the root canal had already saved.
Here is why. A root-treated tooth is brittle — it has lost its blood supply, and between the original decay and the access hole cut to reach the canals, much of its structure is gone. What remains is a thin-walled shell being asked to absorb the strongest forces in your body. A crown wraps that shell and holds it together. A filling just plugs the hole and leaves the walls to fend for themselves. They eventually split — and the fracture often runs below the gum, where nothing can be done except extraction.
A white composite filling makes it worse. Composite shrinks as it sets, and because it is bonded to the tooth, that shrinkage pulls the walls inward — loading the exact structure that is already weakest, and seeding micro-cracks in it. You are asking the most fragile tooth in your mouth to survive on a material that actively stresses it. If your dentist restores a root-treated back tooth with a filling alone, ask why; if it must be a filling, a metal-based restoration does not shrink and does not pull on the tooth.
Most so-called “root canal failures” are not failures of the root canal at all. They are fractures of teeth that were never properly protected.
