Font renderers that cache keep a cache of bitmaps for glyphs that it has previously rendered. Since you need a separate bitmap for each Unicode codepoint or ligature × font size × subpixel offset, the cache could potentially get huge.

So, they cap the number of bitmaps that get cached and evict some. But when you're animating a font's size or dealing with non-Latin languages, you're churning through so many unique bitmaps that you end up not getting much value from the cache.