Of course they could have. Just like they could have done with every other optional or variant rule. But in general they tended to give as core the simpler option, easier for brand new GM and players out of the box and list the tad more complex variants with examples.
Seems to have worked out well.
Seems to have worked out well.
Or they could have just made it the default (supporting it) instead of a variant (as an aside for making logical ability-skill connections). It isn't really a difficult concept to grasp and in products they could list the linked combinations they felt most applicable or appropriate to the task or feature or whatever.