Why is hangout and voice so different in quality?

Just everything about hangout is great. Ability to share materials from other apps, just physically appealing in general, etc.
But Google Hangout has more features aimed for just hanging out.
I like the ability to make calls using my phone normally, with the option to call using my mobile number or gvoice number.
But its messaging features are disappointedly poor.

Blinkonce19 about Google Voice, v2.8