Because of different developments in the sounds after combination:
- 개 ← [가이] ← 가히
- 강아지 ← [가ᅌᅡ지] ← 가히+ᅌᅡ지
Word-internal sounds from Middle Korean often got steamrolled in the process of sound change. Take 고슴도치 (hedgehog) for example. And for a more extreme example, there is the very familiar 괜찮다 which compressed a long expression into its two-syllable stem: 空然하지아니하-.