I'm not sure if anyone here realizes how difficult it is to excel at an elite level in multiple strokes/events in swimming. Besides just a few, actually no, besides Phelps, there are just about zero swimmers right now or in the past two decades who are good enough at multiple strokes to earn medals in more than one. It's difficult enough to be top 3 in two distances of a stroke, much less multiple lengths of multiple strokes. What Phelps did in Bejing alone is enough for me to call him the goat. Winning the 200m free, 200 fly, 200im is honestly just about impossible when I think about how he beat all of the "pure freestylers" and all the IMers (which typically favors breaststrokers). Unfathomable. This is from a former swimmers perspective, but yeah, no question for me.
Why are there so many different strokes is the real issue. Swimmers get opportunities not only through different distances but also different strokes to dominate. That opportunity is foreclosed for those who do different distances in running. Addtionally things like the decathalon that require more multifaceted dominance than swimming only award one medal.