Older boot loaders could only mount primitive file systems so the kernel was kept on boot and the OS on root. One way of preventing some types of failures due to a full file system is to give each thing it's own file system. Another convenience of multiple partitions is one can easley backup and restore them individually. Some file systems like is ZFS, BTRFS, LVM? are not supported by bootloaders that do not themselves use the Linux kernel.
The problem with these ideas is that updating the kernel can fail if the boot volume becomes full when there is plenty of other space available. Fast Incremental snapshots are only avalable with advanced file systems.
So it's up to the OS distribution to decide what trade-offs to make.
Completely independently of file systems there are some features (security and otherwise) that map the kernel to memory then restrict access to that memory with hardware and/or software to enforce rights and mitigate privilege escalation. See user vs kernel mode.