The value of communities in Lemmy comes from the members of the community participating in them. The members choose material to present to their community with intent, with the desire to contribute to their community. The community then adds value by interacting with the presented material (even a negative response from the community adds value to the community by refining the community identity and interest).
Automatically pulling in material from non-community members destroys the value of the community. It pollutes the community space with material that no community member chose prior to its presentation.
The content on Reddit is also submitted by humans (well, mostly).
This assumes that a post pulled from Mastodon to Lemmy will also push comments from Lemmy back to Mastodon (or at least notifications of comments)… which is a real infrastructure problem, and also depends on the Mastodon communities wanting that interaction, and the admins allowing it.