Why are world maps cut off?

I’m afraid that this isn’t strictly a travel question, but I scoured the list of StackExchange sites and couldn’t find any that seemed more appropriate.

When I look at a world map, I always need to remind myself that the equator is lower (farther south) than where I expect it to be. I always chalked this up to some kind of unconscious bias towards the Northern Hemisphere because I live there.

But today I was looking more closely at the latitude lines on a world map, and discovered that there were more northern latitude lines than southern latitude lines! The bottom part of the map must have been cut off, I guess. I clicked through the search results and discovered that this seemed to be ubiquitous. Here are some examples: 1, 2, 3, 4, 5, 6. Some even cut off Antarctica entirely!

I did find a few counter examples, like this one. But they seem to be a tiny minority.

This is extremely disorienting to me. I strongly feel that the equator ought to go through the middle of the map, and that a world map should show the whole world. Why are most maps made this way?