system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can a cactus in the desert live without water for 2+ years, but the cactus on my desk needs to be watered regularly?

I've heard it has something to do with conditions, but this seems counterintuitive since the desert conditions are much harsher (hotter, drier) than those in my house, so if anything you'd think it would require less water.
gpt
Saguaro cactus are the only species of cactus that can go for more than a few months without water.  Very old, very large saguaro *can* live for years without water.  That doesn't mean that all old saguaro will survive - just that some will.  It also doesn't mean that younger, smaller saguaro will survive those conditions.

Most indoor cactus are not desert species - they come from the western regions of the US and Mexico that have a fairly unique climate.  Those regions get tremendous amounts of rain between October and April and 0 rain for the rest of the year.  But just because there is 0 rain doesn't mean there is 0 water in the soil.  The soil only totally dries out for a few weeks in August and September and those cacti store water all year in preparation for that.

Prior to August there is still *some* amount of moisture in the soil and one of the ways that cacti have evolved to deal with that climate is that they will develop root systems that are dozens of times the size of the cactus itself.  Those gigantic root systems allow the cactus to extract enough water from the soil to survive as long as there is some moisture in it.

When people grow cacti they typically give them the exact same amount of space as they would any other plant.  This prevents the cactus from growing its root system enough to survive on soil with a minimal moisture level like it could in the wild. 

If you planted a cactus in an area where it had 200 square feet of soil all to itself then you would be able to just dump few feet of water on that area once a year and forget about it.  If, however, you treat a cactus like you would any other plant - IE, you plant it in a small area and give it small amounts of water at a time - then its going to have needs that are similar to other plants.