>And all the discussion I can find online say breaking a contract doesn't fall under the definition of "illegal".
IIUC, breaking a contract is a civil tort[0]:
A tort is an act or omission that gives rise to injury or harm to another and
amounts to a civil wrong for which courts impose liability. In the context of
torts, "injury" describes the invasion of any legal right, whereas "harm"
describes a loss or detriment in fact that an individual suffers.1
And torts (again, IIUC) are not the breaking of a
legal code, and as such cannot be illegal, as they break no laws.
Something illegal is dealt with in a criminal court (yes, there are administrative courts in various places that handle minor infractions, but those are governed under the relevant legal code and are adjudicated as such.
Civil torts are not contained within any legal code, and as such cannot be illegal. That said, there are some harms that may also rise to the level of criminality (e.g., fraud), which may also have a civil tort associated with them.
But breach of contract by itself is just breaking the rules[1] agreed upon by the parties to a contract.
[0] https://www.law.cornell.edu/wex/tort
[1] https://kids.britannica.com/kids/article/rules-and-laws/6286...