Why would it make sense to not be able to video in 30fps in a 720 resolution, and have to video in a higher resolution to get 30fps? On my GoPro Hero 7 Black, I can choose to video in 720, 1080, 2.7k, or 4k. I can choose 24, 30, 60, 120, or 240 fps.

I wanted to take a video that I didn't need to be able to slow down, and did't need super high quality, so I tried to set it to 720 at 30fps. However, it bounced back to 60fps and told me "30 FPS is supported at 4k, 2.7k, and 1080."

I can understand not being able to video in the higher resolutions at a higher frame rate, such as not being able to video in 4k at 240 (no matter how cool that would be). But why would 30fps in 720 be a limitation? Is it somehow more difficult to video in 720 at 30 FPS?

This also applies to 24fps, BTW.
720 is typically captured at 60fps, not always, it's not required (different video capture devices have different abilities), but 720p60 is more common than 720p30, so it may simply may not be, and likely is not a supported format. 

You also shouldn't have to speed it up (as it should play back at it's base frame rate). This is common on higher end camera systems such as RED and ARRI, where you can set your base frame rate of say 24fps, but record 240fps thus it would play back at 1/10th speed. 

The base frame rate is encoded into the file's wrapper, and most if not all programs would recognize that for correct playback.