It can't be done client side because most of the time the client has no idea about specifics. For example, the client knows you have Minuet buff, but nowhere client-side is it indicated how much attack you got from it. Another buff example, all food is displayed the same way on the client (just the food icon) and the server takes care of all the stats calculation. There's other things, as well (hidden effects and latent effects, etc.).
Anyways, I don't think packets would be the issue here (have you seen inventory packets?), but more likely "we have to actually dedicate someone to moving stuff around in the UI to display these somewhere and update them on packets received".
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