There are a few theories as to why Google has yet to roll out a dark mode for its app. One possibility is that the company is waiting for the release of Android Q, which is expected to have a system-wide dark mode. Another possibility is that Google is still working out the kinks with its dark mode implementation. It's also possible that Google simply doesn't think there's enough demand for a dark mode option.
In the meantime, there are a few workarounds that you can use to get a dark mode-like experience in the Google app. One is to enable night mode in your device's settings. This will tint your screen a dark shade of gray, which can reduce eye strain. Another workaround is to install a third-party launcher like Nova Launcher, which lets you customize the colors of your apps.