Welcome to the forums @Uncool_McUncool!
I think one reason most projects do not rely on Godot's built-in physics is because for something relatively simple like Tetris, it is often easier to quickly roll your own implementation instead. It also gives a bit more control over alignment and precision. Finally, and likely the most likely reason, is because of the need to know whether a row is filled or not requires a grid, it isn't too much extra work to quickly write a simple collision check using the same grid. Since all of the data needed for collision already there in the grid, it might just be a little easier to quickly bash out a collision system through code than to work with Godot's physics system.
That said, I mostly do 3D work with Godot, so I'm just making educated guesses. I've never tried to make my own Tetris like game. It is quite possible there are other reasons they use their own collision code.
