Obviously, you can’t give them code that legally belongs to a past employer and they’re not allowed to look to avoid accusations of copyright infringement.
Especially if they do any reverse-engineering for interoperability, there must be zero suspicion that they were inspired by code they’re not allowed to use.
This is where open source contributions under permissive licenses come in.
Something shown to work in a real project is also viewed better than out of context code snippets.
When you’re essentially saying you have nothing to show them, you’re indistinguishable from someone who actually has nothing and is lying about their skills, so the onus is on the interviewers to vet you, which for various reasons may not be possible, so they’d rather just move on to someone with a clearly proven track record.