I think it just seems that way, because it takes effort to explicitly pull all those arguments into our consciousness, to explain a decision that was taken subconsciously. The subconscious is often equated with the emotional or the irrational, but there is no reason to suppose the subconscious is incapable of rational decision making.

Looking at it from that perspective, you are not rationalizing an irrational decision when you explain your reasoning: you are simply remembering/recreating the process that lead to the initial, perhaps even rational, decision.

To my knowledge, the famous experiments of Kahnenman and Tversky have proven otherwise. When studying the effect of "anchoring", for example, they would actually explain to their subjects how their rational decisions are influenced by this irrational heuristic. Yet the subjects could not rid their decisions of that influence.

Which showed that there are probably irrational heuristics involved in every decision. However, it doesn't show that irrational heuristics are important, or even dominant, in every decision.

On the Wikipedia page for 'Anchoring', there is an example about thinking of a number and subsequently bidding (experiment by Ariely). What happens when you tell people with a high number that they are likely to make a relatively high bid, so they should try to make a relatively low bid? Do they make 'normal' bids or extremely low bids?

This shows that saying people can't rid their decisions of that influence is an overstatement. When you are consciously aware of the heuristics and the decision is taken over a period of time, you can downplay them.

Most studies that I'm aware of in behavioral psychology (granted, it's a young branch of science) confirm my point, though. When they tell people about the anchoring heuristic, people still continue to make similar mistakes.

One way to explain would be to think about the number of variables we should account for in virtually any everyday decision. Finding even a fairly optimal solution would consume too many resources and evolution has taught us to deal with this using emotional shortcuts. On the evolutionary time scale, the 50 years when the question of choosing a programming language was relevant would be a tiny dot.

On the other hand, rational knowledge can by absorbed subconsciousness eventually. But when it does, we become unaware of it, by definition.