You can leave early if you turn 22 in the calendar year of the draft. So for the players in Caitlin's class, those born June-December 2001 would be eligible to enter the 2023 draft (after their junior year) because they will turn 22 that year. Caitlin was born in January 2002, so she won't be draft eligible until 2024, which is after her senior year. So no, Caitlin cannot "leave early." Paige Bueckers's birthday is October 20, 2001, so she could leave after her junior year if she wants.
An interesting case in that class is Stanford's talented center Cameron Brink. She was born on December 31, 2001, so she is eligible to declare for the draft in 2023, after her junior season, if she wishes. Had she been born just a few hours later on January 1, 2022, she would have to wait until after her senior year just like Caitlin.
A lot of times you will see a player leave early if the draft that year is particularly weak, because that means they might have a better chance getting drafted high and making a roster. This is what Charli Collier, last year's #1 pick, did. You get a higher salary if you are drafted in the lottery, so had she remained in college, she'd be leaving potential $$ on the table. Other times you might see a player leave early if a bunch of talented players in the class above them graduate that year. Maybe the team made a final four run and will be in rebuilding mode the following year, so the junior opts out and decides to go pro. This is what Jewell Loyd did in 2015 after helping lead Notre Dame to the national title game that season. Loyd became the #1 WNBA draft pick that year.