Most TVs and monitors have a fixed refresh rate, often 60 Hz these days.
If a game runs at, say, 48 FPS, some frames would be doubled every second, and some won't. The resulting video would seem more jittery compared to 15, 30 and 60 FPS, where frames can be doubled or quadrupled evenly.
That's why FreeSync and G-Sync monitors are a big deal to enthusiasts, they avoid this issue for their refresh rates are exactly what's being rendered.
But every modern TV today are not at 100 or above ?
While TVs with 120Hz output have existed for quite a while already, TVs that actually accept 120Hz input only came out since 2018. Before all they did is take a 30 or 60Hz input and calculate in-between frames to pump up the output to 120Hz.