There’s no particular technical reason why this couldn’t be done, they just don’t provide an option, because it doesn’t really make much sense. It’s either 60 for battery life, which has been a standard in displays for a long time, or the smoother one, which is usually adaptive, to give a balance between visual experience and battery life.
Motorola Edge 40 in particular supports 3 modes – power saving 60Hz, adaptive up to 120Hz, and high performance 144Hz.
Adaptive mode will automatically lower refresh rate to 60 or even less (24 while watching a movie, looking at photos or a static page for example) for power saving, but will immediately switch to 120 when there’s any motion displayed for smooth visual experience. Manually setting refresh rate to 90 or 87 would get you overall worse experience, it’s not as smooth as 120, and uses more power than 60 (24), so there simply isn’t any point in giving the user an option. But if you care about visuals much more than battery life, you can set it to constant 144Hz for absolutely best visual experience (on your phone at least, mine only supports up to 120).
You can actually check how the phone is adapting the refresh rate, and you’ll notice how quickly it switches between 24, 60 and 120, to do that you need to enable Developer Options and then toggle Show Refresh Rate.