Why do some athletes not play for their actual countries?

Something that really frustrates me when I look at certain sports teams is how many players they have who are not really the nationality of the team they play for. Two examples are pictured above. Manu Tuilagi, who is completely Samoan, nothing to do with England whatsoever, not even born there, but just because he's lived there a certain amount of time he gets the right to play for the English rugby team. There is also Danny Welbeck, who is completely Ghanian (he is born in England) and yet he plays for the English national team.

I really don't want to seem racist here, but this is incredibly unfair on the smaller countries like Ghana and Samoa. They produce talent, and yet it gets taken away from them by the bigger national sides. This then affects their performance on the pitch.

There are many other surprising examples of this: take for instance the New Zealand rugby team. Let's run through some of the great names of "New Zealand" players. Jonah Lomu has two Tongan parents, and spent his childhood in Tonga, yet he is born in Auckland and so can play for NZ. Tana Umaga has Samoan immigrant parents, and like Lomu, he is only eligible to play for NZ through being born there and having lived there. Joe Rokocoko is Fijian. Born in Fiji to Fijian parents, only migrated to New Zealand aged 15. And there are many more examples of so called "New Zealanders", like Keven Mealamu.

There are so many cases like this across sport, it's really frustrating. How different would it have been if Jonah Lomu had played for Tonga. They would be a rugby power, as Lomu's talent would not doubt have attracted other good Tongans who would otherwise have played for New Zealand.

If Manu Tuilagi had chosen to play for Samoa at the world cup, he could have partnered his brother in what would have been a scary backs lineup.

I've been focusing too much on rugby (it is the most affected sport), but other sports also have this feature. Eoin Morgan in cricket for example, plays for England but he is totally Irish and like all the others only gets his dual nationality through residency. Elena Baltacha in tennis plays for England but all her family is Ukranian. The list could go on and on.

It's obvious that all these athletes are just searching for success, but it could have been so different. If one of these people decided to stay loyal to their actual nationality, arguably many would have followed, and this could have built up the smaller country into a good team in the sport.