system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why does England compete in some sports but participate in others as Great Britain or the UK?

I'll try to add some context. 
So in the Olympics, they compete as Team GB with Northern Ireland instead of being called the UK. 
In Soccer and Rugby, England, Wales, Scotland, Northern Ireland all compete separately with their own National teams.
Someone like Rory McIlroy is from Northern Ireland and is represented during the PGA with a Northern Ireland flag but represents UK but called British.
gpt
Association football was invented in England. It spread first through the UK, so the first games between countries was England vs Scotland, but Ireland and Wales got their own teams around the same time. 

Later, when FIFA first started the World Cup, each nation's football association (FA) was invited. The UK had 4 different FAs (one for each country) so they joined individually. There was once talk of combining to one team in the World Cup, but the French were adamantly against the idea since a combined UK team would be too good (at the time) 

These days, the nations of the UK are too well entrenched behind their national teams to ever consider playing on the same side. 

A similar story goes for Rugby. 

Things get a bit more confusing because Northern Ireland and The Republic of Ireland used to be the same country when these sports were invented. Soccer is the exception here, most other sports are played under a United Ireland. 

Cricket is a bit older, and was invented back when Wales and England were the same country in practically every way. The England Cricket Team represents England *and* Wales internationally.