What vSphere version?
DRS is very different in vSphere 8 compared to vSphere 6 or 7.
In any event, memory balance would not be the only factor for DRS to balance workloads, what about CPU and Network load? And even if DRS determines that an improvement to the DRS score in vSphere 6/7 could be improved by migration, the low migration threshold (level 2 from your description) might preclude these being actioned because the determined amount of improvement to the DRS score after the move is not sufficiently large enough to warrant it.
If your hosts are lightly loaded even with all work loads hammering away, i.e. you have little or no over-provisioned resources you might not see any migrations at all.
In Sphere 8 DRS changes to a different model, and rather than working to a DRS score and trying to maintain a good 'balance' in the cluster, it works out a 'happiness' score for all VMs, and evaluates whether this score could be improved by moving the VM to a different host. Again if the VMs are getting what they need where they are, then they won't move, no matter how 'imbalance' the cluster may appear to you.
You say that your cluster is under no stress or CPU and RAM, which leaves Network IO. Do you have some very chatty VMs with high network IO, that DRS may be trying to separate onto separate hosts?
