You are still creating the base in your legs. Years and years of training will create a solid foundation upon which to build.
Think of it this way. Doing base, you will put the foundation down for a house. Those with years of the sport in our legs have a pretty solid foundation for the house. Going to build too early will have you create a house on not as solid of a foundation – so when things get tough, the house can shake easier and have bad things happen to it. (Imagine a house on sand – walls are there, but underneath does not really support them).
So by focusing on base, you may have weaker walls, but they are more likely to survive the shaky test.
Those that have spent years creating the foundation can spend time creating stronger walls so it is harder to create that shaky environment.
I am short on time, but I hope this makes sense.