Because their country doesn't offer the same opportunities to achieve the things they believe they can in their new country. It's the main reason people emigrate. No one's going to build Nvidia in India or China, for instance, at least not back in the time that it was created in America.
Kind of ironic, considering the whole original population of America (the whole continent) was mostly replaced by immigrants, starting some time after 1492 AD.
The British India Company colonisation of India was institutional colonisation.
The company remained for many generations but most (not all) individual British people stationed themselves in India and retired 'home' to England where to whence the money flowed.
Many European countries regarded their "colonies" as places to exploit for resources, places to be travelled to for work for many years, places they didn't regard as 'home'.
There's more to "colonisation" than just simple migration.
Even in Australia the early colonisation was by settler families that were part of larger stay at home families, the black sheep, wayward sons, and fortune seeking adventurers .. in Australia convicts and servant families remained, ultimately a good proportion of the underwritten (from home) settlers stayed .. but initially at least many heads of these families "returned home" to England.
In a similar manner modern USofA has many institutional "colonies" around the globe, bases they've established and never left, these rarely have permanently settled multi generational families.
That's false. By definition none would have come if there wasn't opportunity. You don't need to be a geography student to see that it wasn't just irreligious opportunity.
That's not how it works. It depends on whether (how much) they are paid and if so what do they buy with that payment. Are they settled in US buying pizzas and Netflix?
Because if not, then investments to Nigeria past $x will grant it the upperhand since all that money can get you another doctor and more.
5 years is truly a big kill. looks like its a reaction to 99% of them refusing to send money back home and invest in Nigeria, preferring to buy-US and -Europe.