for inferred regulons, their transcription factors do not present in dataset?

[Polligator]

I have some questions regarding how GENIE3 works in SCENIC, hope someone can help me out.

My understanding is that GENIE3 tests the correlation between the expression of all genes and known transcription factors present in the dataset. Thus, for the regulons finally identified by SCENIC, the corresponding TF should be present in the dataset as well, however, it appears not to be the case. Many of the TFs are not even expressed in the analyzed dataset, just wondering how does this work exactly?
Thanks very much!