it isn't

An extra chip means higher cost to produce & assemble board, larger board size, more pins wasted on this nonsense, most fast-edge signals to route, more passives, extra risk to handle for one extra chip being out of stock, and it is much easier to extract firmware than even from a "protected" stm32

Also wasting RAM (and power for it) on code, or random (between high and very high) latency of XIP from SPI flash