system
Given the following post, respond as if you're commenting on Reddit:
human
[Pokemon]Why do so many trainers not have a full party of 6 Pokemon? Also, why can't you carry 7 in the first place?

I understand why Mikey the preschooler with his first Pokemon might not have a full party, but what about those training under gym leaders? What about gym leaders themselves? It almost seems unfair when I walk up with a full party of assorted types stacked up in a way that I can almost always pull a type advantage with same type attack bonus against whatever type your gym specializes in...
gpt
Nothing is stopping you from carrying 7 or more, but official League-sanctioned matches cap at 6 (and you have to declare those six before the battle starts). Most pokegear and belts and stuff come with 6 slots as a result of this, so nobody bothers carrying 7 or more pokemon around with them.

The reason most people have only a few pokemon is that raising pokemon is *hard*. You probably *own* enough for a full team of 6, but if four of them are level 3 pidgey and rattata you're not going to get very far. In order to train 5 or 6, you have to be training full-time. Most people can't afford to train full-time, and anybody funding themselves purely through betting money will run out of money the first time they tilt (losing 3 or more matches in a row). The one exception is breeders (since breeding makes a fair amount of money), and they usually have a low-level team because they have to split their attention between each pokemon and don't care much about winning matches. If you have a day job or even a part-time job, you'll find you just don't have the time to give them all the attention they deserve. It would take a seriously dedicated and talented trainer to manage a working team of 6 at a professional level.

As you pointed out, gym leaders do have the time and resources to have a large team. Most gym leaders have a dozen or more pokemon they're training at any given time. Go ask a leader you defeated for a rematch and they'll bring out a balanced team of 6 level 45 pokemon to crush you. However, the League's guidelines suggest that most leaders give out a certain number of badges each month, so they run the weaker type-specific teams you're familiar with. There are a couple of notable exceptions, like a certain dragon-type gym leader who prides herself in having an impossible-to-win badge.