Fonts begin as vector graphics, but every single character on your screen isn't rendered as a tiny, individual vector graphic. Text rendering libraries use a lot of optimizations to cache rasterized glyphs because they're reused all over your screen.

It also gets extraordinarily complex when you have to handle text layout, kerning, languages that read in different directions, styles, and all of the other complications that come with font rendering.

TrueType fonts even support a small virtual machine to process tiny code programs related to font rendering. It's an incredibly deep topic.