Yes, this has always annoyed me too. Also, Plate providing a Higher AC isn't much of a justification. Scale and Plate are one point apart, one point, yet with Scale Spec you get +14AC and no penalties and with Plate you get +15AC and -2 check and -1AC. That's with PHB armor, but the Adventurer's Vault items mitigate this somewhat. High level Scale and Plate have the same AC, but Plate does get Resist 5 all for Tarrasque Plate. Although, that comes at a cost of it only being +12AC armor, or +13AC with armor spec. So you're sacrificing 2AC points to be at the same AC as similar level Scale armor, and you get a -2 check and a -1 to speed.
The additional kick in the pants is that Scale and Plate require different abilities. If you're going for a heavy CON Defender, you're kinda screwed, because Scale requires DEX of 15, whereas Plate requires CON of 15. So if you wanted to go Scale Spec to get rid of your penalties, you can't, unless you sacrifice some of your CON and/or WIS.
But you're comparing different tools for different jobs.
Alright.
Plate armor and Scale armor are for two different character classes.
Plate armor is not what your fighter is wearing by default. So stop glamming onto it and thinking it is what all fighters want. It isn't. It isn't -made- for fighters. This is an artifact of third edition creeping in there. Abandon it.
Paladins -are- the high AC class. They also have ranged powers as well as a ranged mark so they don't need to be mobile to operate. They don't go to the enemy all the time; sometimes they just bring the enemy to them, as their mark is -designed- to work that way. So Paladins can suck the movement penalty and the penalty to skills that they don't use so often (relating to mobility, see the pattern) and they can behave, instead, like a divine champion.
Fighters have to be more mobile. They have to go to the enemies, or use the rare power that brings enemies to them to be at their most effective. A fighter's mark on a non-adjacent enemy is just a mark and nothing more. Only adjacency makes the fighter's mark a threat. They -have to be more moble- and that's why they don't go all the way up to Plate Armor.
Beyond that, there's two builds of each, in terms of defenses: With or without a shield.
Fighters with a shield can choose: Armor specialization, or Shield specialization. Do they want to have no penalties to their mobility and skills, or +1 to Reflexes? They get a choice.
Paladins with a shield don't have as good a choice: Shield spec is clearly optimal. But that's okay, because Paladins can -still- get that additional point of AC despite the requirements of Plate Spec. They don't need mobility or skill ups, so why worry? Plus, they're ahead of the Fighter anyways, so -shouldn't- have 'moar' made easily available.
Without a shield, each has less of a choice, but then you're not a defensive build anyways, so AC isn't your primary concern. If it were, you'd have a shield, obviously.
What you're bumping your heads against is the fact that you're trying to stick an armor-type onto Fighters that isn't made for them. It's another class's tool. Why -would- it be better for Fighters?
That's like wondering why there's little healing Scale, or mobility based Chain. Wrong class. Wrong tools.