Recently, the experimental discovery of high-Tc superconductivity in compressed hydrides H3S and LaH10 at megabar pressures has triggered searches for various superconducting superhydrides. It was experimentally observed that thorium superhydrides, ThH10 and ThH9, are stabilized at much lower pressures than LaH10. Based on first-principles density functional theory calculations, we reveal that the isolated Th frameworks of ThH10 and ThH9 have relatively more excess electrons in interstitial regions than the La framework of LaH10. Such interstitial excess electrons easily participate in the formation of the anionic H cage surrounding the metal atom. The resulting Coulomb attraction between cationic Th atoms and anionic H cages is estimated to be stronger than the corresponding one of LaH10, thereby giving rise to larger chemical precompressions in ThH10 and ThH9. Such a formation mechanism of H clathrates can also be applied to other superhydrides such as CeH9, PrH9, and NdH9. Our findings demonstrate that interstitial excess electrons in the isolated metal frameworks of high-pressure superhydrides play an important role in generating the chemical precompression of H clathrates.