We've taken the most obvious latency path a normal user sees and somehow decided that mess was better than sending an ar file.
(Not that this wasn't inevitable as soon as someone decided CSS should be a text format... sigh)
If you're talking about data: background images in CSS, then all the CSS parser has to do is find the end of the url(). It doesn't have to do any base64 decoding or anything like that until the rule actually matches.
For what it's worth, in my profiles of pageloads in at least Firefox I haven't seen gz-decompression of stylesheets show up in any noticeable way, but I can believe that it could be a problem if you have a lot of data: images in the sheet...