Why dose my iPhone turn off by itself

By turning off you mean the screen turns off and can be awakened by swiping up on the screen, or that it turns off completely, and must be restarted by holding the SIDE button for 10 seconds until an Apple logo appears?

I suppose that you mean the screen turns off, or in other words, the device goes to sleep.

If you want the screen to be on at all times, a simple settings change will suffice.

1. Go to Settings
2. Go to "Display & Brightness"
3. Select "Auto-Lock"
4. Change it to "Never"

I hope this helps.