Reasoning:
1) The current marker icons available to mission makers by default is very limited (https://community.bistudio.com/wikidata/images/f/f0/A3_MarkerTypes.png) and too military focused for them to be useful in most mission scenarios.
2) The only way for mission makers to get custom markers is by making a mod but most mission makers don't want to go though all the work that making (and maintaining) a mod implies and how to distribute them to potential players.
For confirmation just search the steam workshop for "custom markers" mods and they are very limited in both their numbers and also their content.
Yet mission makers would love to be able to greatly improve the immersion and map readability of their missions by being able to have a simpler way to customize the look of the markers they use. Currently they aren't even able to access some very cool icons already contained in the Arma 3's data files.