Or, just a thought here, it's because FSR is open source. You can literally go look at it on github right now.
DLSS is not. Guess which one is easier to implement into a game? If you guessed FSR, you'd be right. You don't need to involve AMD the company at all to implement FSR into your game. That is not true of DLSS and Nvidia.
You're taking a selection bias as a causative argument from a conclusion.
DLSS being closed source is literally an example of Nvidia's anticompetitiveness, by definition.