I suspect it’s an architectural decision. The simplest way to handle a change (add/modify/delete something) is to simply start from scratch: restart the system and process everything from the beginning.
OpenHAB works the same way. Only difference is it monitors its config files for changes. If it detects the file has been altered, it automatically restarts and loads it. It is convenient but can pose a possible problem if you made syntax errors. HA lets you ‘check config’ before committing your changes.
Both systems have low ‘granularity’. In other words, they only see things at the level of a file (its all or nothing). They don’t understand that you only added one sensor or made one minor change (sensor’s name).
FWIW, the home automation system I’ve used for many years has high granularity. I can add a new driver (a.k.a. component), a switch, change the name of a sensor, add automation logic, or just tweak a single line of an automation, without ever having to restart the HA system. In other words, there’s just no reason to restart it. Perhaps some day Home Assistant will work this way as well.