You are right that someone could buy the physical book and them torrent the digital version. But I would consider:

1 - You already forfeited you digital sales because you decided not to have an ebook release. If you make public that there are no plans for a digital release, and not the case that's just delayed compared to print.

2 - Digital books tend to be cheaper. Makes a huge difference for people that read 10 books a month.

3 - Print takes space, which many people might not have. I'm not going to buy a print book at a higher price, just to have to go straight to a used book store and sell it there, so I can torrent the ebook - which would be illegal anyway.

Same goes for TV shows and movies. Having dozens of blu-ray+dvd combos is not feasible.