I don't disagree with you at all in principle but last year no one offered in excess of £15m for Clarke and the deal was poorly structured from our perspective from what I understand. The club didn't feel that was near enough his value to entertain and I'd be inclined to agree. Whilst clubs have shown interest in other of our players, I'm not aware of any concrete offers that are anything other than derisory (the exception, particularly with the benefit of hindsight, would perhaps be Southampton's offer for Roberts).
Meanwhile the money Middlesborough got for Akpom, I would say was very satisfactory from their perspective and the money for Spence was complete dreamland stuff. Coventry got a decent deal for Hamer and an amazing deal, considering he had a year left on his contract, for Gyokeres.
So what do you propose we do differently? Reassess and lower the valuations we put on players (I don't think many of the other clubs you use as an example of good practice do that)? Actively tout players around to attract interest?
As I say, I agree with your general point but don't see that it's generally in the club's control or should be used as a stick to beat them with.