The way they're internally implemented doesn't allow pinning an IP. They buy a rotating proxy service from a vendor, and don't get to choose their source IP.
If the source IP was an issue, you could do it other ways: for example, make the cookie rotate on every access, and insist there is a single stream of accesses.
You say “they” as if all scrapers are a monolithic group with the same constraints and goals. Part of the problem is the massive diversity.