Why didn't OSes securely delete files right from the beginning? And why do they still not do this?

Because of the following reasons:

• Performance - it takes up resources destroying files. Imagine an application that uses hundreds or thousands of files. It would be a huge operation to securely delete each one.
• Extra wear and tear on the drives.
• Sometimes the ability to retrieve a file is a feature of the OS (e.g. Trash, Recycle Bin, Volume Shadow Copy).
• As noted by Xander, sometimes the physical storage mechanism is abstracted from the OS (e.g. SSDs or network drives).

Instead of another "You are wrong because" answer I'd like to take a slightly different approach:

Early computer OS's were written by programmers for programmers. Any one who programs and knows what pointers are understands that "deleting" a pointer doesn't delete the thing its pointing at: they are separate.

That doesn't mean that delete doesn't actually delete. That pointer is gone. Trying to use it after "deleting" it (freeing the memory, rebinding the name) can result in bad things happening.

But history marches on, and now end users who have a different concept of delete (like yourself) are in the picture. They (and you) have expectations that are not unreasonable (whatever else is said in this thread).

But delete will not ever mean (for a computer) what you think it should: there are reasons both technical (detailed quite well in other answers) and social (45 years of inertia).

The modern (and I'm including *nix) OS abstracts a lot of things for you: you no longer need to be a computer expert to own/operate a computer in the same way you no longer need to be a mechanic to own/operate a car. The price you pay is that those abstractions are leaky: there's a fundamental disconnect that can never quite be bridged. A computer "document" isn't really a document, a "desktop" is not a desktop, a "window" is not a window, etc.

It doesn't have to be corrected because it's not a fault.

The pointers to the file are deleted, and the area the file occupied is marked as free space. The drive then overwrites this area in its own time. It's purely there to save wear and tear on the drive. After all, storage devices (especially SSDs) have a limited number to times they can write before they fail. Most users would not appreciate their drive failing after 6 months.

Secure solutions do exist with tools available to securely wipe free space on a hard drive.