It takes a long time to get truly skilled in something. What we call an expert is really what a reasonably intelligent person can be expected to learn about a single topic within a lifetime. The gains in one area compound on themself. Given that these gains compound, you can't learn 2 things half as well. So the T-shape represents the constraint that you only have a shot at becoming an expert at 1 thing. Spend 80% of your time on that and 20% on the rest.

The Pareto principle (80/20 rule) also implies that you only need to read a handful of books on a single subject to know more about it than most people. The knowledge gains become marginal unless you focus most of your time on it. So the things that you find interesting that aren't your main thing, just "learn the ropes" and move on back to your area of expertise. That's the most efficient traversal of the Sum of Human Knowledge: Familiarize yourself with the lay of the land, then focus most of your energy on some place that hasn't been mapped yet.

This is a complex topic!