I don't really see how that's the case, generally.
Melissa and Grant are leads, which is more taxing than doing an ensemble show, and Grant's been in this role for nearly seven years, which can't be said for anyone else left on any of the shows but Caity.
Despite rumored tensions between some people, I haven't seen any indication anyone else from The Flash is eager to leave. It hasn't lost too many cast members over the seasons and those they have have been the show's decision, except when Keiynan decided he didn't want to be tied down to anything for too long, Legends of Tomorrow included. Likewise, all of the cast exits in Arrow's first five seasons were writing choices, with the exception of Colton needing a break for personal issues; he was happy to return once he was better. Most of the later exits were the actors' choices, and mostly of cast members around since S1, although I'm unclear if Paul asked to leave, and there's conflicting stories about what led to Willa's exit. Batwoman is new and Black Lightning's had very few cast changes and no rumors of unhappiness.
That leaves Supergirl. To my knowledge, only four actors have asked to leave or chose not to renew their contracts.
Over on Legends of Tomorrow, three of the first four exits (Wentworth, Arthur, Victor) were definitely the actors' choice. I thought Ciara's was purely a writers' decision for years but recently rumblings suggest it might have been a mutual agreement. In any case, she has no desire now to ever return, unlike plenty of other actors across the Arrowverse who either chose to leave or were let go but are happy to make return appearances as their lives/schedules and story permit.
It's my belief that all signs point to Franz's exit not being his decision, but generally fans waffle between being adamant he asked to leave and being adamant it was a decision the producers made, depending on which argument is being employed to make the producers look good. I do recall that after initially saying he was open to returning in whatever capacity, he later hinted that he'd moved on and wasn't particularly interested, but that was nearly two years ago now.
Keiynan, as noted, wanted a break from acting altogether and to not be tied down. Do we know if Ramona was let go or asked to leave? She booked another role immediately after leaving.
There were indications Nick was unhappy being in Vancouver late last season that certainly had a lot to do with his family, but that's no different than Echo asking to be written off Arrow to spend more time with his family or Melissa's rumored reasons for supposedly wanting off Supergirl.
There's also been rumblings based on Matt's own comments to fans that he's just doing this to try to get a Constantine show back off the ground.
Brandon and Courtney wanted to stay, but Courtney was only on for two and a half seasons, and that first season she was recurring. Comparing her situation to that of Melissa, Grant, or even someone like Emily doesn't make much sense.
In fact, more than half the remaining cast have three seasons or fewer under their belt, versus The Flash having a majority still around since 2014 and an additional cast member who made sporadic appearances starting early on before becoming a regular a few seasons back.
It also helps the Legends seasons are shorter than most of the others.