This is a very good question. There are a few reasons that could explain this:

1. Habit and familiarity: Many older Rails projects used Feature Test instead of System Test, so programmers are used to this approach. Switching to using a new System Test requires adaptation and learning.
2. Backward compatibility: As mentioned, System Test is only supported from Rails 5.1 onwards. Many older projects still run on old versions of Rails, so System Test cannot be used.
3. Incomplete understanding: Not all Rails developers clearly understand the difference between Feature Test and System Test. Dissemination of information about the advantages of System Test has not yet been fully implemented.
4. Requires more complex configuration: Compared to Feature Test, System Test requires a bit more complex configuration, which may discourage some programmers from using it.
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