We do have this information - and for the record I can confirm that it was marked "invalid" by a moderator - but this information (at current) is only available to the development team (not ♦ moderators - although they can see what flags you raised).
Re providing more verbose feedback - unfortunately the numbers make that prohibitive. There are enough flags going through the system that asking moderators to annotate each and every flag is not practical (it would be asking too much from ♦ people who already give a lot).
We do, however, show many other additional actions that have happened following a flag - deletion, (public) moderator comments, etc - so you can usually see that a flag has been followed up on.
We could probably include the "valid"/"invalid" (which we only store against flags-for-moderator-attention, btw) - but I'm a little in two minds whether this is beneficial.
After discussion, we have decided to defer any decision on this for now; simply, we don't want flagging to be a distraction. As soon as you start showing this on a flag-by-flag basis, we can expect to start having to defend each and every decision. That really isn't somewhere we want to go, on something that a: tends to be subjective, and b: is an overhead on our much-valued ♦ moderators; even picking from a drop-down of explanations would be (in our opinion) unacceptably expensive on other people's time.