That’s because you’re only tweening the PrimaryPart of the model, moving it do not also move all other parts into the model.
To move an entire model and all the parts it contain, you have to use :PivotTo() on the model itself, but it is unfortunatly not supported into a tween.
So the only solution for you is to weld all the parts together, and tween the CFrame of the PrimaryPart instead of its position and orientation.
You can use this plugin to do it easier, click on the PrimaryPart, then Ctrl + click on all other parts of the model (so they all are selected in blue), go to the plugin tab of your studio, and click on the “Weld All” button.
I think you could use PivotTo using cframe values. The downside with this is that the animation can look laggy. Here is an example on how you could use PivotTo:
local model = workspace.insanemodel
local cValue = Instance.new("CFrameValue")
cValue.Parent = workspace
local coolmovingtween = tweenService:Create(cValue, tweeninfo, {Value = CFrame.new(model:GetPivot().Position + Vector3.new(-24, 0, 0))})
cValue.Changed:Connect(function(val)
model:PivotTo(val)
end)
coolmovingtween:Play()