The simple answer is its hard and slow so you make a compromise.
More often than not you can find a appropriate upper bound for resolution on your target platform and rasterise ahead of time into texture atlases and the like.
You could also do this at runtime to support arbitrary resolutions but memory becomes a concern at a point. And including a rasteriser for your vector graphic format of choice may not be appropriate.
Basically using bitmaps and hoping nobody looks real close is a compromise you hope nobody notices.