Why is Android rooting not as fragmented as iOS jailbreaking?

Magisk is not comparable to an iOS jailbreak, because Magisk simply relies on an existing possibility to access a device on root level so that it can access and modify the flash memory directly (e.g. on an bootloader unlocked device via custom recovery ROM or an existing su/root access).

Instead iOS jailbreaks first have to execute complicated exploit chain just to get root access. Furthermore current jailbreaks are non-persistent, which means the bootloader and the kernel can't be permanently changed. Therefore an iOS jailbreak requires way more effort for reaching a same "state" at run-time that Magisk provides on Android.

This means in the end the jailbreaks have to be adapted to the iOS version and every jailbreak also has some disadvantages depending on the way the device is exploited.

Such problems are unknown to Magisk and therefore Magisk is way simpler to install.