I think I know why posts get less upvotes than comments.

Studies suggest that approximately 90% of people are right-handed. Left-handedness is far less common than right-handedness. Studies suggest that approximately 10% of people are left-handed.

Since most of the user in this subreddit have vaults open, I can assume with some confidence that they're using the reddit app on their smartphones. Most smartphones nowadays have 5-6 inch (12-14cm) diagonal length.

The width of the phone averages around 6-8cm

The average length of our thumb is 4.9cm (35-65mm)

The upvote button for posts are located on the far left corner of the screen. Thereby making it impossible for your thumb to reach it without lifting or adjusting your grip. So it takes more calories to upvote.

But the comment upvote buttons are relatively closer to the middle of the screen. So it takes less calories.

So, people aren't upvoting posts because they're conserving energy.

Disclaimer: it's a light hearted joke xD