A recent example of this I couldn’t help but notice is I have this beautiful updated Swaylock screen SGS made on a fresh install I put on a new laptop. 
There are several factors that contribute to this “feature drift”. One is that after the initial installation, .config/ and other directories below the /home folder are not typically modified during updates. The home folder contains the user’s personal files after all, so this is probably for the best–but as you noticed this also means that updated config customization does not “trickle down” from new ISOs in any appreciable way (unless you restore from the updated skeleton).
Package selection changes in the ISO would similarly be way too intrusive to try to push into existing installs. It’s fine for a maintainer to say, “Pamac sucks lately, let’s take it out of the ISO and use Octopi,” or whatever–but if you start barging in on someone’s standing installation and ripping out software that they might be using to put in something else that you think is better, people are bound to become unhappy with some of those choices. Once the user is has their installation up and running, it’s probably best they make those kinds of decisions for themselves.
You can read through the changelog that the Garuda team posts on the Gitlab page if you are interested in what is changing from ISO to ISO. It comes up easily in a web search, or find it here. You can make package changes on your own if you spot something that appeal to you, and updated config files sometimes get conspicuously posted so you can grab those as well.
Really the only way to ensure you are enjoying all the niceties of the freshest ISOs is to develop a backup routine for files and configs that will allow you to periodically reinstall without too much disruption, and then once or a few times a year go ahead and refresh.