Well,
technically, inaccessible cardinals can be written as aleph-k for some inaccessible cardinal k, and the same holds for... pretty much any large cardinal property, really. The issue is that large cardinals, by their general definition, are literally "too big" for ZFC (why do you think they're called
large cardinals?), and while the consistency strength of large cardinal axioms =/= the size of the large cardinals that observe them (huge cardinals seem to be smaller than supercompact cardinals, for instance, despite the existence of a huge cardinal being a "stronger statement" than the existence of a supercompact cardinal.) Simply put, the large cardinal hierarchy is kind of just nonlinear.
Anyway, I don't have like, a mathematical proof behind what I just said. It just seemed intuitive to me, but I remembered that large cardinals can be expressed in terms of aleph numbers - it's just that most avoid doing that because it's not useful.
Now, I do wanna ask: from where do you get your claim that inaccessible cardinals can't be proven to be larger than anything in ZFC? An inaccessible cardinal, at least for our purposes, is a cardinal number k with the following properties:
- Uncountable: k is larger than aleph-0.
- Regular: k cannot be reached by the union of less than k sets which are all smaller than k.
- Strong limit: k cannot be reached by repeated power set operations.
Given all of this, I'm not sure I understand how something comparable to inaccessible cardinals in size can be proven by ZFC without any additional axioms.