To make your comparison apt, though, you would need to allow these characters those options, and then have them just not work at random some percentage of the time for an arbitrary reason.
If your complaint is about the % chance mechanically, then you can gripe about concealment, too. And I believe they largely exist for the same reasons: so that even at high level, they are still very relevant.
If you're saying that arcanists get hosed because they just have a random failure chance instead of something like a non-proficiency penalty, regardless of what feats they pick up, I'll point you back at the reasons that they likely did that: the robed wizard archetype, the fact that most wizards shouldn't really need armor anyway, and the fact that wizards represent the design extreme of a Paper Tiger, so they must be very powerful in one area and weak in most others.
So why should you randomly screw up spells while you're wearing armor? In order to be proficient in armor you've got to multiclass. Why is that not cost enough?
For this particular post, I'm not arguing balance. I'm arguing reason. It doesn't make sense as is.
Well, in-character, the explanation is "detailed, expansive, complex somatic gestures that are required for arcane spells that are not required for divine spells." Complete Mage has a few in-character theories for why this might be, such as divine spells are entrusted by a force, but arcane spells must be pulled from reality by the caster herself.
Out-of-character, it's an archetype thing, a balance thing, and a need thing. Merlin didn't need mortal metal, and Gandalf never wielded a shield. Wizards and Sorcerers are supposed to be "soft" targets for most effects. And melee is usually a sub-optimal choice.
As for the ranged attacks...
Don't need armor except to go into melee??? Ranged attacks roll against AC too, and arcanists have a way of drawing fire. Let us not forget those little buggers readying an action to disrupt their spells. Also, many spells require a mage to be within pretty close range to the target; you're not getting that scorching ray or lesser orb spell off from a safe distance, I'll tell you that from personal experience.
For ranged attacks, wizards have access to things like
invisibility,
entropic shield, and even taking mundane cover or "soft cover" behind the fighters. And unless something has 20' reach, you can get off those rays and orbs from a pretty "safe" distance, at least as far as putting a meat shield between you and your target is concerned. You don't always have a fighter between you and the enemy, but usually you will, and it is a tactical descision that is really encouraged.
It makes sense from those three perspectives alone. There's no disconnect in allowing Clerics to cast in armor, but not allowing Wizards, Bards, Sorcerers, and the like. I'm willing to bet the *strongest* reason is archetype. Wizards rarely get their own hands dirty. They hire adventurers for that.
