Why don't I have GFCI on my outside receptacles?
You've answered your own question:
the code changed since then.
The code has changed since 1992. GFCI wasn't required anywhere back then and, in '92 it was pretty darn new technology that was just rolling out.
Note: it was pointed out in a comment that the NEC started recommending GFCI for outdoor outlets in 1975.
Remember, though, that the NEC is simply a recommendation and that it's up to each state to ratify and adopt some version of the NEC as its state code. Some states modify the NEC for their own purposes and some cities/counties make further modifications (such as Chicago requiring conduit in residential construction which isn't an NEC requirement, nor, AFAIK, even an Illinois state requirement outside the Chicagoland area). Other states do adopt the NEC unchanged but are very slow to adopt new versions. For example, Indiana has adopted the 2008 NEC basically unchanged, even though there have been many updates to the code since then.
TL;DR: Your state may not have adopted GFCI requirements for outdoor outlets when these were installed. Or, if they had, these may have been put in by a homeowner who didn't want to spend the extra for proper replacements and didn't know/care that it was a code requirement.
You're right, your outlets have worked just fine for the last 30 years, and will, most likely continue to do so for another 30 years. Also, the technology has improved and pricing for GFCI has dropped dramatically in the last 30 years as well.
Should you add GFCI protection to your outdoor outlets? Sure, why not? It's not going to hurt anything and it will improve safety.
I see in comments on your question that you're planning on replacing all your outdoor outlets with individual GFCI outlets. I guess you're made of money, because they're still not cheap. One GFCI outlet, installed correctly, will protect all the outlets downstream of it. If you feel the need to replace the outdoor outlets, by all means, do so, however, replace them with a decent "commercial grade" non-GFCI outlet for $3-5 each and leave the $30+ GFCI outlet indoors. There are a dozen or more questions here with info on how to do so, search for "GFCI line load" and you'll probably find the proper instructions.
Also noted in one of the comments on your question is that it's generally recommended to install a GFCI outlet as the last one in the chain inside the house and use it to protect all the outdoor outlets. Indoors is much better weather protection than the "weatherproof" covers for outdoor outlets, and adding "weather resistant" to the list of features on the receptacle only increases the cost of the GFCI further for limited additional life.
Finally, you could replace the breaker for the circuit that supports the outdoor receptacles. This would put GFCI protection on everything on that circuit, including, possibly, some indoor receptacles, possibly lighting, and whatever else may be plugged into that circuit. For lighting, GFCI is not required and can be a bit of a danger (a circuit trip from outside leaves the lights off inside), or bad for an appliance that's plugged into this circuit ("why did the fridge just turn off??"), so this may not be the best option.