I think the answer might be ‘because of marketing’.
For me, I dislike both the power button AND the screen fingerprint reader. The power button because I like to access my lockscreen and not to be catapulted in the last app I was using ; the screen reader because it’s messy and oftentimes doesn’t work correctly. My newborn has been able to unlock my phone, multiple times…
What I really miss was the fingerprint reader on the back of the device. It was convenient, fast, precise and let me access the lock screen.
If you downloaded the game illegally then installed it on your hard drive then… You have your answer.
You might still have the downloaded game and you have the installed game. Depending on the fiability of your download source there might also be temporary install file, somewhere.