When we are young, if we put our heart and soul into becoming excellent at something, that comes with a cost of not learning something else. At age 15, 20, or 25, if we become a star athlete, we simply do not have the hours available in our day, week, month, or year to also become a star musician. If we master the guitar, violin, and six other instruments, we probably are not simultaneously attending med school. If we become a surgeon by age 30, we probably didn't also write the world's next great novel or win gold in the Olympics.
This logic applies not only to accomplishments, but also to skill sets. If we go deep into STEM and become a top engineer or software programmer, we probably didn't also have the time, natural ability, or inclination to also become a skilled entrepreneur or salesperson. Indeed, STEM-focused people are not typically known for their abundance of interpersonal and communication skills, and pro salespeople don't typically moonlight as biochemists or astrophysicists.
If we do somehow manage to do exceedingly well in two areas, inevitably something else has to give. Perhaps we don't have much social life, or don't have time for romantic relationships, don't exercise enough, or don't sleep enough.
There simply isn't time to learn it all. We make choices and do the best we can in the areas we don't focus on. It's often said that it takes at least 10,000 hours to master a skill. The original researchers who said that actually said it's probably 20,000 or more. Working 40 hours a week, we have only 2000 hours in a year. If the 10-20K hours rule of thumb is true, that means it takes at least 5-10 years to master a skill if we work at it full time, which many of us can't afford to do given additional responsibilities of school, work, families, and so on, meaning it could take even longer than 10 years for many people.
However, we can begin to defy the Zero Sum Learning Law sometime around age 30, relative to other people anyway. This is because many people stop learning, or at least slow the pace way down. We settle into a single job, a single industry, a small and defined set of interests. We stop reading. We become exceptionally busy with life, pursuing a career, starting a family. This isn't inherently good or bad. It's just an observation that very few people continue to climb steep learning curves throughout their lives. Polymaths who never stop learning are rare to find, but they do exist.
These folks who do keep learning, growing, and pushing themselves continually add whole new skill sets, experiences, and expertises to old ones. They eventually explore the intersections of their various interests. These people learn to see beyond the limitations of their initially chosen disciplines. These people become visionaries and leaders. These people change the world, locally and globally.
Up through our 20s, we simply haven't had time to develop all aspects of ourselves, to master more than one field, skill set, or way of thinking. In our 30s, 40s, and beyond, provided we never stop learning, we can all become true polymaths and change the world.