As for my second point, those programs you mention (except for maybe Unity) are used by more seasoned developers that have a much firmer grasp on things and wouldn't want or need a feature like that.
Although honestly those programs should have a feature to do that still because why not? I wouldn't think implementing a command that basically amounts to copy and paste is hard or time consuming.
Whether something is hard or time consuming isn't all there is to deciding whether a feature should be added to a product or not.
Implementing such a thing isn't as much "fire and forget" as it may appear to be. Whenever you implement a feature, you also implement the need for maintenance and support. If this feature was as dead simple as you described, chances are high that those who use it will eventually wish that it could do more. As a result, they might file support tickets, which need to be handled by support staff, might be forwarded to the developers, and who knows, maybe they'll even implement a bit more than what was originally planned. Otherwise, it still takes time out of someone's day to discuss whether the suggestion should be listened to or ignored.
That would mean that the super simple copy and paste just turned into something that most likely took more than a day or two to discuss, implement and support.
If these suggestions all get turned down and the feature remains as just a basic copy & paste, which still has to have graceful error handling in case the program lacks permissions to write to a certain directory, or the user lacks them, or there's no space on the storage drive... this may leave a bad impression of the developer, as they are not listening to improvement suggestions of a feature that is obviously lacking. Now the addition of such a simple feature has led to damaging the company's image.
None of the outcomes above are desirable for the developer, the product or its community as a whole, and these are just a few examples of the side effects of sloppily implemented, badly maintained and close to irrelevant features. Using the proper tools for the job, rather than a low-effort reinvented wheel, should take precedence over convenience for a limited subset of users.
@Monchan: Since it hasn't been mentioned yet, your only hope to get it back at this point is file recovery software. Due to the nature of file deletions, the more you've used your storage drive in between the deletion and now, the lower the chance of successful recovery... but it's there.