Because, you have to properly clean electronics to get a quality life out of the product.
Example:
If you use AJAX, and scrub all the thermal compound off, you will still leave residue of Chlorine and Pumice on the chip. No telling what emissions you will be breathing while the PC is running.
If you use Windex- again who knows what will be left on the chip. Sometimes it may act as an agent slowly eating away at the top metal surface, or heavens forbid, using it on a Centrino where the ceramic/glass surface is exposed to those residuals. Then one night when you got home from partying and left the computer on, doing a throrough virus scan- the PC goes up in smoke, taking all your hard drives with it
Again, that is why forums exist, so that reliable information and experience can be passed along rather than assuming 'other stuff' would work just as well.
================================
Even a broken clock is right twice a day