This is just the same principle as private organization boards of directors delegating the minutia of running the organization to the executives and their teams. If you think it would be madness for hiring decisions on individual contributors to be made by board votes, then you should support the delegation of rule-making authority to executive agencies.
Yes, it means that changing the executive might change the rules. Congress remains free to overrule the agencies by passing further legislation, if they so desire. And voters remain free to replace the executive the next time around, if they want to see different rules. These are all features, not bugs.
There is certainly value in stability and predictability, but there is even more value in having an executive branch of government that is empowered to make decisions quickly and a short feedback loop between the public and the government.