SpaceX has a different design philosophy than most other launch providers. Most launch providers optimize for performance efficiency, SpaceX optimizes for cost efficiency.
example:
The Ariane 5 uses different engines for their first and second stages while the Falcon 9 uses the same engines for their first and second stage. Using different engines for the lower and upper stages is pretty standard among launchers because having one engine designed to operate in the lower atmosphere and one engine designed to operate in the upper atmosphere/vacuum leads to better performance and is one of the reasons the Ariane 5 high performance delivering payloads to GTO relative to the Falcon 9. However, from a manufacturing perspective it is much more expensive to build two different engines than it is to just use one type of engine which is part of the reason a Falcon 9 launch is around three times cheaper than an Ariane 5 launch.