ESXi is a purpose-built thin hypervisor that is by intention reduced to the functionality that is needed to run VMs, preferrably in an enterprise environment,
And ESXi is NOT derived from the Linux kernel (although it partly uses the same POSIX sub systems like many other Unix derivates), so adding Linux features is not just a copy-and-paste of code.
If you really want to combine multiple disks into one VMFS volume then you can use VMFS extents to achieve this. But software RAID functionality is just not built-in.