If placeholder support is provided by default in UITextView like UITextFiled then it will reduce some overhead from developer. Several time it is required to show some placeholder in textview. Just to implement this either we need to subclass textview or we need to write a lot code to handle this.
The Swift.org project and swift-evolution have no control over which features UIKit offers. Those sorts of higher-level frameworks belong to Apple.
If you'd like to suggest changes to Apple's frameworks, you should file a problem report in Radar at <https://bugreport.apple.com>. Most likely somebody has asked for this feature before, so your report will be marked as a duplicate, but reports with many duplicates are more likely to be prioritized.