When you use an encoding based on integral bytes, you can use the hardware-accelerated and often parallelized "memcpy" bulk byte moving hardware features to man

When you use an encoding based on integral bytes, you can use the hardware-accelerated and often parallelized "memcpy" bulk byte moving hardware features to manipulate your strings. But inserting a codepoint with your approach would require all downstream bits to be shifted within and across bytes, something that would be a much bigger computational burden. It's unlikely that anyone would consider saddling themselves with that for a mere 25% space savings over the dead-simple and memcpy-able UTF-32.