Two things to note in regards to parts: First and foremost, you have to consider age of the model, not the date in which you purchased it. For example, your model may have been replaced by a newer model 5 years ago, thus your model was already no longer in production. And second, if they only supply parts to repair centers, then they likely only plan on supplying parts for the length of the warranty (typically one year).
In addition, repair shop rates (like your local mechanics) are getting $100+ an hour (the last time I checked a couple of years ago, it was about $125), plus a diagnostic fee (which is sometimes rolled into the repair cost), plus parts. So even a basic repair could set you back close to, or over $200. Most people won't spend over half the cost (or more) of a new unit for repairs. Depending on your unit type and cost, most manufacturers don't even perform repairs, they simply replace as it's cheaper. No reason to keep a bunch of parts on the shelf which may never get used, which would then be a waste of money (ie: loss of profits).
So while I agree with you as a consumer, I understand why they operate as they do as well.