The issue is that the transaction and the outlooks are asymmetric. The purchaser is often playing a longer game than the seller, so the seller doesn't often have the luxury you mention - given the constraints of the types of companies involved in such transactions.
Consider the board on which you have posted - Often, the purchaser is acting over a greater time frame, and the seller has an immediate need. Competitors are at your heels, and you can't realistically enforce patents against the purchaser or competitors, while retaining the market agility that is required.
