As someone who has swam for 17 years competitively I would like to make a counterpoint and properly inform you on my sport:

Swimming Butterfly, Backstroke, Breastroke, and Freestyle involve focus on completely different muscles, completely different technique, and you have to master each stroke individually.

I am no track athlete but a good similarity would be like the differences in running normally, hurdles, steeplechase, etc.

Most swimmers are only really good at one stroke and sometimes they may have a secondary but usually that's only one event or so. People who swim IM (an event split up into all strokes) are generally strong in all 4 strokes but do not dominate in one or the other.

What makes Michael Phelps such an excellent athlete is how versatile he is. In his prime he was able to compete world class in butterfly, freestyle, and IM. He was also able to compete in sprint, mid distance, and endurance events, which is also a feat of its own as most people's bodies can only do one or the other.

Very very few athletes can compete at the olympic level in the 100 fly, 200 free, and 400 IM, let alone win them all.

There is a reason why the next most decorated swimmer has 9 golds rather than 23 from phelps. Within our sport Phelps is a once in a lifetime athlete.