Rules Engine only allows you to use the Contains operator with Values, not Fields:
It would be very helpful if we could use the Contains and Does Not Contain operators when using fields fields because there are some cases where we want to look at a larger string or text area value and see if it contains or does not contain another field. For example, if I had String A and String B and want to see if A contains B.
I can see this being useful for filters as well as case expressions.
If there is a current workaround for this, I would love to hear about it!
As@sai_ram mentioned, this is because of technical limitations. We do not have any plans to enhance this product at this point in time.