The Shui Family sisters arranged to meet together today because it is their time to challenge the Trial Arena; such opportunities are rare. For Golden Core cultivators like them, it happens only once a year. Win, and you get to continue; lose, and you're out, waiting for the next year.
As a Big World with a Cultivation history spanning hundreds of thousands of years, the Three Cave Realm is a very mature Cultivation Realm Domain with a developed cultivation civilization permeating every aspect of life. The number of Golden Core cultivators in the Three Cave Realm exceeds a hundred thousand, so without restrictions, the Trial Arena would soon collapse. So, although it seems like a random challenge, there is in fact a limit to the number of attempts.
This is the same in the other Seven Realms, where everyone treasures this rare once-a-year opportunity.
