I'd imagine we don't want to set a precedent.
We are very consistent with having a valuation for players and typically not exceeding it (if buying) or accepting much lower (if selling) and this stance probably helps us a fair bit when negotiating transfers, or dealing with clubs/agents in general.
If we suddenly pay £40m for a lad who only has a year left on his deal, then we likely have difficulty taking advantage of such situations in the future.
If our valuation is, let's say, £32.5m - then I can understand us not wanting to be seen to "just pay the extra" given his contract. A bit different when they have 3 years left on their deal.