No technical reason really, it's about cost, demand and volume.

Cost is going to be a problem because of the high current involved in normal operation and even more so in case of a short-circuit.

Let's take a popular battery size, let's say 2200mAh 3S 25C. This battery should be able to safely support a discharge current up to 55A. For reference, a 10KW power shower draws less current than this from the mains network. Our smart battery controller needs to be able to measure the discharge current, and needs to be able to switch the output on or off.

You can just buy a hall effect based current sensor and a power transistor that will handle 55A. But these components need to survive in case of a short-circuit for long enough for a micro-controller to read the current at its regular polling rate and then turn off the transistor. Expect the short-circuit current to be a number of times higher than 55A.

So you end up with a parts list that costs you about $10 per unit for small quantities (500). Have I mentioned the price of the battery I was building this for? It's $13.

I've thought about building some for myself, but 1) they really should be mounted inside a hard case together with the battery and 2) I don't have a space where it would be safe to prototype this, assuming that a failure will happen.