Two reasons: first one needn't ship any external bitmap files. Second I can easily change the size of the font, for e.g. title, score etc. It's just for lazyness if one uses my interpreter so he/she needn't take care of that. Just a "drawtext" and that's it. But no one is forced to use the built-in font but can use his own bitmap font.
In my old version of EGSL I am using SDL_gfx which has a built-in bitmap font and I recognized that a lot of people use it for a simple text output (and even input).