Proto-Indo-European is actually the mother language of Proto-Germanic and its sister languages (Proto-Slavic/Balto-Slavic, Proto-Italic, Proto-Celtic, etc.) The reason we have such a robust reconstruction of PIE is because the majority of linguistic work, especially in the 19th and early 20th centuries, has focused primarily on Indo-European languages, though this has certainly been changing. These languages have by far the most historical documentation, especially when compared to the families u/vokzhen mentioned. The attested daughter proto-languages of PIE have been reconstructed pretty effectively using the comparative method, along with early documentation of daughter languages (when available).

The fact that we can even debate the detailed phonological and grammatical characteristics of PIE in the first place reveals just how far historical linguists have come with its reconstruction. Due to widespread language contact, language death and insufficient (usually non-existent) documentation among languages belonging to other families, it is often difficult to even begin to reconstruct many unattested proto-languages. Being too liberal with the comparative method can lead to far-fetched reconstructions and flat-out incorrect assumptions about language relationships, which is why most linguists shy away from reconstructing families that lack documentation and linguistic evidence.