"I have worked with a few embedded SDKs (by Microchip, Keil and Espressif), and all of them have a really dumb built-in implementation of the heap."

Many embedded systems allocate just once at startup and these "really dumb" implementations are completely sufficient. The vendors provide a fast, low-overhead solution by default and trust that developers can go find a more appropriate allocator if they need it for their application. Luckily we have many implementations to choose from!