Only because software RAID works amazingly well! And now with SW RAID, when a disk controller goes bad you aren't screwed in cases where the manufacturer no longer makes them.
Does it though? As someone who used MD raid for absolutely everything for over 10 years, were now switching to HW raid. EFI / GPT disks just don't work that well with OS level raid, because the motherboard is well within it's right to modify the EFI partition, which if you have raided at the OS level will cause massive issues.
The EFI partition doesn't have to be RAIDed. All you need in the EFI partition is a stateless, RAID-aware bootloader that is hardcoded to look for a RAIDed volume matching a generic label (such as "rootfs") and then chainloading the actual bootloader from there. Then you can just install this "shim" bootloader into each disk's EFI partition once and you don't need RAID or to even mount it ever again.