> there is already a swastika unicode glyph

It's a Chinese character and there are two variants. The left-hand variant (卍) represents 和 (peace) in religious contexts, whereas the right-hand variant represents 力 (power). The specific glyph the Nazis co-opted is the right-hand variant, and it is considered inappropriate to use in the West (for this reason, I am not reproducing the character here).

The glyph appears in texts predating the existence of Nazi Germany, and I assume that is the reason the Unicode Consortium has not removed the glyph yet.

Note that I am not defending this decision (nor the usage of the glyph today). One could argue that historians should use a special font that can render these two glyphs, but the problem is likely a lot more subtle than I am thinking.

As an aside: the left-hand variant is used in Japanese maps to mark the location of Buddhist temples.