why does a system catalog insert/update/delete not fire a trigger?

hiho@ll

As far as i know, the reason a trigger doesn't work on system catalog
(e.g. DDL statements) is because it's not a INSERT SQL statement.
It seems 'simple_heap_insert' is used for insert's.
So i thought why not fire a trigger event after the simple_heap_insert
or the index update's?
What's wrong with fire a trigger event after/before the system catalog
insert/update/delete?
Is it impossible cause it affects data consistency in case the trigger
crashes?

If youre asking yourself why i need this:
i already had some cases where this feature would be helpful.
It's just a nice-to-have and i'm interested in the reasons ;)

thx@ll