I’ve played tennis for years in both shorts and skirts. Impeding movement due to shorts has never been a problem for me perhaps thanks in part to modern stretchy materials and 2” inseams. I always preferred shorts because of the pockets and the fact that skirts tend to get blown around by wind. Plus dudes wear shorts and are just fine. I’ve never seen shorts rip in a major tournament and I’ve never seen dudes wear skirts in one either.
Tennis has always been a preppy sport and shorts are seen to this day as casual. Plus the girls probably want to look cute when they are being broadcast internationally. That said, I’ve seen the Williams sisters make some pretty poor style choices in various tourneys. Giant hoop earrings and hoochie short skirts come to mind.