People love to hate on RPGM these days, but the realistic answer is accessibility. Older RPGM versions made it more difficult to increase resolution size and early RPGM games mostly came out of Japan where the norm was smaller resolutions because most people in Japan had smaller resolution monitors.
In the modern versions of RPGM like MV, increasing resolution is very easy and can be done within 30 seconds. But, the main reason is most people don't use 1080p monitors today. The most commonly used resolution worldwide is closer to 720p (followed by 1080p) so if a developer wants to reach the maximum audience size, it makes sense to use the most common resolution size.
Here are stats for monitor resolutions worldwide:
You must be registered to see the links
Another thing to consider is playing a game at a smaller native resolution than your monitor can be done easily, but the reverse can cause issues more often.
Personally, as I am a developer for an RPGM game using DAZ renders, I originally wanted to make the game in 1080p then downsized to 720p for this reason. All of my renders are originally done in 1080p anyway and I just use photoshop to scale them down to 720p for the game. Thankfully, Photoshop has a very good algorithm for downsizing images while maintaining quality.
Hopefully that answers your question.