History. The Mac didn’t have “paste without formatting”, and assigned ⌘-V to “paste with formatting”.
Even if it had, they might have made it the default, either to show of that styles copied over, or for consistency. If you paste a picture, you don’t lose formatting, either (of the picture, whatever that means, or any text in the image)
If you copied some text in MacWrite, for example, exited MacWrote, launched MacPaint, and ⌘V-ed into MacPaint, you likely wanted to keep the font, size and style.
Some programs also have several variants of “without formatting”. Excel is a good example (https://support.microsoft.com/en-us/office/paste-options-8ea...). Which one should be the default?