I think first, it's important to understand why publishers ask for a pitch packet rather than a full comic:
- They don't want you to waste your time working for months on something that they might reject. They'd rather give you a chance to show them what you're planning first and let you know if they like it or not before you work on it for months.
- They may want to assign you an editor to help raise the quality of the storytelling through feedback and advice.
- They probably only need to read 5 pages and a quick synopsis to know if the quality or style of your work is what they're looking for and don't feel any need to read a whole graphic novel.
- They might have requirements for things like page sizes they'd want you to work to and would ideally like to give you a template before you start.
- They might have suggestions for changes to the content to shift the target market a little, or just to target it better.
With a completed comic, your best bet is probably smaller indie publishers, because they might be fine taking on a small, independent graphic novel that's possibly niche interest and not drawn to the same technical specs as everything else they make. Though a lot nowadays might be surprised that you didn't just put it online, or go via Kickstarter to publish it yourself.
I don't think it's impossible to just cold submit a graphic novel to a publisher and get picked up, but pretty much everyone I know who made a comic with a bunch of pages then got published, it was because they already published it themselves either online, or by printing and selling it at events and through an online shop, so publishers saw it could sell. That said, even in those cases, it's nearly as common for the publisher to commission that creator to make a new comic for them than to pick up that one.