For the simple reason that teams under penalty who cannot use all their allocations have not been able to deal their slots. Teams with large budgets blowing the cap signed all the talent and did not need to trade for the allocations. Thus there has not been much of a market.
We also traded two virtual non-prospects last year. A non-drafted free agent (Taylor) with so-so stuff and a largely disappointing 4th round pick (Oliver). The price would even be smaller this year. We can also assume that virtually none of the Cubans are "wired" to a particular team especially over an 18-month period where theoretically teams are planning their budget.
So yes, we would trade a couple non-prospects to sign a 16-year-old pitcher for $2 million who was rated in the top 10 international prospects. Every team should do that. The A's, who have much less money than we do, just spent $4 million including the penalty for the kid. You have no evidence for your theory on the value of allocations. And there is a lot of evidence to suggest they have little or no value.
Bottom line is that if the Phillies do not trade for allocations they are missing an opportunity to add more young talent to their system. It is a missed opportunity that would only cost money and probably would be a far better use of money than a guy like Buchholz (where the use of money is also not bad, just international spending is better and more efficient).