Because your ESC use Dshot, that is a digital.protocol, wheras a servo tester send s an analogue pwm signal. You will first need to change the settings on your ESC to turn off dshot., But having Dshot is way better to o suggest you leave it on.
If you need to test your motors/ESC you can do so via software now. Read about that here BLHeli ESC Guide for Multirotors (passthrough mode)