

Much of the data on your phone, including critical information that’s required to run the operating system and make the device function, is fully encrypted when the device is off/rebooted.
While in this locked down state, nothing can run. You don’t receive notifications, applications can’t run in the background, even just accessing the device yourself is slow as you have to wait for the whole system to decrypt and start up.
When you unlock the device for the first time; much of that data is decrypted so that it can be used, and the keys required to unlock the rest of the data get stored in memory where they can be quickly accessed and used. This also makes the device more vulnerable to attacks.
There’s always a trade off between convenience and security. The more secure a system, the less convenient it is to use.
It’s exactly what the reboot process is designed to do; return you to that fully encrypted pre-boot state. There would be no purpose to implementing a second method that does the exact same thing.