NES: Hand-Coded Sprite Flicker

Same reason why ghosts on Ms. Pacman on the 8-bit Atari computer flicker[0] when they overlap horizontally. The flicker is slower so looks more like flashing rather than flickering.

The Atari has 4x 8-pixel wide 'player' overlay graphics that were single color has a horizontal (but no vertical) position, and spans the entire vertical height (so 1 byte of data per scanline), as well as 4x 2-bit wide 'missiles' which could also used as a 5th player. In the case of Ms. Pacman the missiles are used for the whites of the ghosts' eyes.

[0] https://youtu.be/p43aO7JhmN8?t=94

This is known as sprite multiplexing and was used on other systems as well to get around the hardware limits of 'max number of sprites per scanline'.

With the phosphor persistence on old CRT televisions, sometimes the flicker wasn't even really noticeable if it was done right - the sprite would just appear to get a little bit dimmer on-screen.