When players reach level 5 in Pokemon GO and they attempt to participate in a gym battle, they are first asked to choose a team to represent. Players can choose between Team Instinct, Team Mystic, and Team Valor, with each team having its own leader, legendary bird, and color scheme that define it. A driving force behind the gameplay in Pokemon GO is spreading the influence of one's team as far as possible by taking over gyms, which converts gyms to the color of whatever team players picked.
This team dynamic has created serious competition between Pokemon GO players, who want to control as many gyms as possible and make sure their team is on top. People that play the game together will generally want to be on the same team so that they can cooperate to take over and defend gyms, but what happens if someone was playing Pokemon GO near launch, selected one of the three teams at random, and didn't consult their friends first?
Unfortunately, anyone that would like to switch teams to play on the same one as most of their friends is out of luck. As of the time of this writing, Niantic still hasn't implemented a way for players to switch teams after they select one at the beginning of the game. While this discourages players from just switching teams all the time to put Pokemon in gyms, it also makes it impossible for some people that regret their team selection from being able to work cooperatively with their friends. And if people haven't been able to work with their friends to take over gyms, they may have been discouraged from playing the game altogether.
All things considered, it's odd that Niantic hasn't offered some way for players to switch teams. It seems as though the biggest reason why Niantic hasn't allowed players to switch teams is because it is afraid people will abuse the system, but there are some easy restrictions that could be put into place to prevent that.
For example, Niantic could make it so Pokemon trainers are only able to switch teams once within a certain period of time. Whether that's once a week, once a month, or even once a year is up for debate, but a simple time restriction would go a long way in preventing people from switching teams all the time. Another way Niantic could discourage people from switching teams too frequently is by putting it behind a paywall; perhaps the first team switch is free, but then players have to use in-game currency or real cash if they would like to switch teams again. Or maybe players could be punished for switching teams by losing all the Pokecoins that they've earned in gym battles as their previous team.
Of course, there could be other reasons why Niantic hasn't allowed players to switch teams in Pokemon GO. One thing that comes to mind is that the three teams will become more important in future updates, perhaps giving players bonuses or special items depending on what team they selected at the start of the game.
Another possibility is that the legendary bird associated with each team will be in a special raid battle that is only accessible to team members. So Zapdos could only be caught by Team Instinct members, Articuno would be exclusive to Team Mystic members, and Moltres would only be available to those on Team Valor. This would mean that players would have to trade Pokemon to catch all three legendary birds, which could be Niantic's way of encouraging players to use the trading system that will be added in a future update.
Team switching isn't something Niantic has spoken at length about, so all of this is just speculation at this time. We really aren't sure why Niantic hasn't allowed players to switch teams in Pokemon GO, especially since it could be keeping some players from fully enjoying the game. There are no real advantages to being on a team other than one's friends, and since some people undoubtedly selected a team at random back in the day, it seems like letting players switch teams at least once would only benefit the Pokemon GO community.
Pokemon GO is available now for iOS and Android mobile devices.