Can anybody tell me why the Magus and the arcane archer do not get them?
because magic.
Whoever wrote them didn't want them to have it. There's not a "reason" other than they don't
The Corrosive abilities didn't exist when the Arcane Archer was written. For the Magus, I suspect the decision to leave them out was based on power balance. You can get both Corrosive and Corrosive Burst using the Magus FCB if you are a Gnome.
So acid is some how stronger that fire, cold or electricity?
As I remember a designer's argument from a related discussion, it's rather about book dependancy. If you want to play a magus, you only need the CRB and UM - not even the APG which predated UM (note there are no APG weapon properties for the magus). Well, despite a few spells that were introduced in the APG, but you will be fine with CRB (and UM) spells.
While it's technically all freely available in the official PRD, not everyone enjoys digging through several books for a single thing and not everyone enjoys using online resources (due to readability, ban of technical devices at the table, being old-fashioned etc.).
Finally, a magus doesn't need corrosive that urgently:
1) Acid is pretty good for a elemental damage bonus, if you have to decide once and forever. But a magus can add bonuses on the fly - and there are some foes which are vulnerable to either fire or cold. Flaming and frost become way more attractive in such situations.
2) A magus can add corrosive via crafting. Actually the class is pretty good at crafting: Full CL (unlike ranger, paladin etc.), Int based, bonus feat that can be item creation feats. It's just that you usually have other priorities.
3) A gnome magus can pick it up as FCB, as noted by Gisher.