It really isn't that much though. A 1200x1200 DPI monochrome image on Letter size (not even considering margins) paper is on the order of 16 MiB uncompressed. And bitmaps of text and line art compress down heavily (and you can use a bitmap atlas or prerendered bitmap font technique as well). It’s also usually easier to upgrade RAM in a printer than a crappy firmware.

> most printers still render fonts and such internally.

Many printers have some scalable font rendering capability, but it is often not usable in practice for high fidelity. You absolutely can raster on the host to either a bitmap font, or make use of the PDL's native compression. Most lower end printers (which is pretty much the bulk of what is sold) do not have the capability to render arbitrary TrueType fonts, for instance. A consumer/SOHO level Canon laser using UFRII is going to rely on the host for rastering arbitrary fonts.