We do? Why? Explain please.
Without the influx of new cultures and ideas, we become stagnate. This has been the curse of many burgeoning countries. Look at the countries that once flourished and are now without the bloom of their youth (UK, Germany, France). Once the input of new peoples ceases, death of their country follows thereafter.
(Will admit that my argument only bodes true if Islamic cultures are excluded.)
The US is a great example. Think how great this country would be if we had excluded the Germans, Irish, English, etc.? All their input was tantamount to making this country great.