IANAL, so disclaiming any sort of legal advice here, but the contributions a founder would make in helping to collaboratively create a new company -- everything from a proprietary idea to business model to naming to market entry strategy to logo to features and functions to contributions to code -- go far beyond copyright. With circumstantial evidence pointing to collaborative creation, and no agreement to the contrary of joint ownership (e.g. a "work for hire" contract) or any agreement specifying specific proportions of ownership, a court would be reasonably likely to rule that OP is a partial owner of the entire company. (Whether it is worth the cost of winning this claim is another matter entirely.)

Whereas copyright is the more limited right to own a specific expression (e.g. writing, images, code syntax) in its exact or near-exact form.

Here is the form of agreement you might sign during the formation process if you decided, after a startup weekend, to jointly form a company:

"Developer hereby assigns to the Company exclusively throughout the world all right, title and interest (whether or not now existing) in (i) the subject matter referred to in Exhibit A (“Technology”), (ii) all precursors, portions and work in progress with respect thereto and all inventions, works of authorship, mask works, technology, information, know-how, materials and tools relating thereto or to the development, production, use, support or maintenance thereof and (iii) all copyrights, patent rights, trade secret rights, trademark rights, mask works rights, sui generis database rights and other intellectual property rights and all business, contract rights and goodwill in, incorporated or embodied in, used to develop or produce or use, or related to any of the foregoing ((i), (ii) and (iii) are collectively “Intellectual Property”)."

As you can see it includes copyright but is a superset.

[edit: formatting]

Thanks, I thought you were suggesting copyright wasn't part of this set.