Some good answers here already.
I'm going to try to take a slightly different angle: it sounds to me as if you're questioning the risk assessment of your test team.
In my experience, "it's the same code" can be a valid reason not to re-test something. It can also be that the code written by your team is exactly the same, but when deployed to a different environment, possibly with different libraries, with things running on multiple servers instead of one, talking to various third-party sites directly instead of mocked out, your deployment is flaky, etc - that it will behave differently. So, as others have pointed out, there can be various good reasons to re-test. The important question is, how do you figure out whether the risk is worth the cost?
When I decide whether I need to re-test something in a staging environment, these are some of the questions I ask to help me figure out risks:
- What environment differences are there, that we know of, between test and staging?
- Is this particular area (or areas that might affect this one) different? If we're not sure - how likely do we think it is that it'll be affected?
- How important is this particular functionality? What's the worst that could happen, if I don't retest and it doesn't work?
- Whose opinions did I ask for the previous questions? Did I miss someone important?
- (MOST IMPORTANT) What other tests would I not get time to run, if I do this testing?
How much visibility do you have of their decision making? How much do they have of yours?
If you think they may be making the wrong decisions, or you just don't understand why they're making those decisions, I would first suggest you try an informal chat with them to understand what affects their assessment of risk, before offering any suggestions or criticisms. It may be that it doesn't make sense because it's political, not technical (tread carefully if that might be the case). It might be that there are issues you aren't aware of. It might be that things that are very easy for you to check, aren't easy or aren't visible at all to them - and you could help out there.