Because while the country exists, the economic wheels must turn and we still need workers to work the factories and offices and citizens to pay the taxes?
Its not always an issue of "us versus them" or foreigners stealing jobs... There are pros and cons in all things and in this, I tend to be more liberal and open.
I've worked with Indians who are now PR and the only thing that stops them from holding SG citizenship is that they cannot hold dual citizenship. The Indian government is happy to recognise the dual citizenship but they cannot do so for SG because SG will not grant it.
Sure, u can call it opportunism or anything u want, but which self respecting professional is not ambitious and opportunistic these days???

Many foreigners I've met who are interested in citizenship are well heeled individuals who will be in a position to contribute through $$$, brains, network... We should not think them unworthy.
1st generation will always have one foot in their original country of birth, 2nd generation will be more rooted and 3rd generation may not even remember where their great-grandparents came from.
As for those who "just meet" the criteria and accept citizenship, let us not forget that many of our grandparents or great grandparents were in that situation once too... And many of us turned out ok and are contributing to SG today.