Why should an athlete practice speed and change of direction when they can just lift weights and then go play their sport?
Speed and change of direction are skills just like many other skills involved in sports. If an athlete learns and practices these specific skills it will set them up to be much more effective at acceleration, top speed, and changing directions quickly.
The problem with just leaving it up to the sport to teach is that each sport has so many of its own unique and specific skills that must be taught that there’s no time for speed and change of direction, and they become simply a prerequisite for the sport.
This is why speed and change of direction are such important skills to learn and practice outside of sport so that when an athlete goes to practice they’re ready to go.