Growth marketing managers are in high demand right now, and it's a challenging role to keep. There are pressures on both sides.
If you hire a growth marketing manager and they underperform, fail to grow your business, or even hurt your business, then you dump them and hire someone else. They've been unable to perform, and they aren't necessarily bad marketers, but they're a poor match for your business, so you need someone else. On the other hand, if you hire one and they do very well, chances are they'll be adding that success to their resume. Talent scouts and large organizations are constantly scouting candidates on LinkedIn, and there's a possibility that they will poach them within a year or two. Growth managers have a high turnover rate because the bad ones get fired, and the good ones get poached.