The short answer here is that tag blocking was not designed as an integral part of the tag system. It's an add-on, intended for rare problems that cannot be handled any other way. That may not remain true forever, but that's how it is right now.
The long answer is that there's effectively no difference between burninating a tag and... never using the tag in the first place. Oh, there are lots of little details that vary, but if you imagine a scenario where someone creates a tag by using it on a question and then immediately removes it within the grace period so there's no record of it in the revision history... Well, that tag will be deleted by the system at the end of the day, and as far as the system is concerned it looks exactly the same as an intentional mass-removal (burnination) would.
The end result? A lot of tags get deleted, for all sorts of reasons. Blocking all of them would be a lot of overhead at best, and potentially harmful at worst. For every tag that gets removed with prejudice, there are easily hundreds that are removed because they were simple typographical errors... Or simply don't have enough questions to warrant keeping around.
One might imagine a system where blocking - and burnination - work rather differently: instead of wiping all traces of the tag from the system, the tag would merely be marked either "inactive" or "prohibited", and either ignored or ignored and blocked as a result. In such a system, blocking would be a superset of burnination, and any tag that required conscious effort to remove would naturally qualify for it.
But, that is not how the current system was designed, nor is adding such functionality a trivial extension of what exists today. When we talk about "blocking", we're not even referring to a system specific to tags - rather, it is a system intended to block harmful or malicious input from users, completely divorced from the tag system. Indeed, the most common tag that are blocked are tags that never existed: each new site comes with a block list entry for that site's topic, thus preventing the [chess] tag on https://chess.stackexchange.com/ and the [rebase] tag on https://worldbuilding.stackexchange.com/ (don't ask).