Psychology. Humans are loss averse, and furthermore they tend to see things in the moment. It's very hard to agree to give up a player who's is good right now for a number on a draft board with no name or face. A lot of fans would legit choose Anthony Mantha, a good but not blue chip prospect, over a top 10 pick in this year's draft, just because of he has a name and nice measurements.
Understanding the value of an asset like Kessel seems vaguely similar to me to understanding the time value of money, depreciation, and other financial matters. Most people utterly fail at these things.
People need to look at it this way: Kessel's career is half over. So what you're actually trading is at most 0.5 Kessel-careers, not 1.0 Kessel-careers. But he's also got a large cap hit now, so the 0.5 of his career remaining is the less valuable half of his career - the first half where a player is rights restricted is much more cap friendly. So you take off another 0.1 or 0.2, and you're left with 0.3-0.4 Kessel-careers as his value.
That's why if we're offered the 5th pick straight up for Kessel we should take it and run, despite Leafs fans' protestations of "hurr durr, there's no guarantee Strome will be as good as Kessel". Well no ****, you're not doing the deal expecting the pick to be as good as Kessel. You're doing the deal expecting the pick's career to be at least 0.4 as good as Kessel's. If you hit that mark, the trade is a steal. And obviously other teams know this too, so there's no way such a trade happens.