Because it is trivially easy to follow the rules in 3e to get to such an end point and what is important in both 3e and 4e is the utility of the end point not the exact explanation of how you get there.
I think both matter. I agree that what 4e has taught me is exactly that...the utility of the endpoint is what is important.
But everything in the 3e books kept telling me the opposite. It wasn't important what the end AC of a monster was. It mattered that they were built on the same rules at the PCs so as to keep things fair. I bought into it completely. OBVIOUSLY it was balanced. Because everything the PCs could do or get, so could monsters. If the PCs can scour books looking for that feat to give them just ONE more point of AC, so could the monsters.
It made sense for an Ogre to have an AC of 13 because they were big and slow and rarely wore armor. This balanced out their large strength and hitpoints. It also made perfect sense for that Ogre to get an AC of 21 by putting on Full Plate. After all, if the PCs could do it, it was balanced for the Ogre. The game told me that was perfectly fair...even if an 8 point difference in AC could be enough to cause a TPK. It might be a little cheesy...but it was legal.
What you really meant by unfair was significantly tougher than baseline. It doesn't really matter how trivially easy in 3e it is to follow the 3e rules and get an AC of 36. It also presumably wouldn't fly in 4e to say "It's a level 10 monster, it has appropriate defenses for it's level. There's no rules for what AC it can have. This one is higher than normal." since "if you come across some custom monster that manages to have 10 more AC than every other monster anywhere near its [CRLevel]...well, it feels like it isn't fair anymore. Of course you're going to lose....even if it IS legal."
I agree. But the difference is that in 4e the rules say "Use this table, adjust it one or two points away from the baseline, but never more than that or you'll likely kill everyone."
So, I expect every monster I fight to be within 1 or 2 points of the baseline in 4e. And yes, you are right, I would call a DM on a monster who was 10 points above normal.
In 3e, I didn't expect any particular AC. I knew that it was perfectly legal and plausible that a monster had an AC of 30 as a CR 5 creature. Sure, that meant that no one in the party could hit it except on natural 20s. It was cheap...but fair. As long as the rules allowed it, we were playing the game the way it was intended. Can't complain about that. Well, we could complain about that if the DM just made stuff up. After all, it probably isn't immediately apparently how the monster got 30 AC. But if the DM can say "He's got a shield spell up because he had an Ioun Stone borrowed to him that lets him cast a spell out of it. Plus, his Full Plate was enchanted to +3 by his 14th level cleric ally who ran off before you got there." then you can say "Oh...we lost badly...but it was fair because he had the help of a powerful Cleric...of COURSE we lost. And we could have done the same thing back to him if only we had a powerful Cleric ally."
It meant that some combats were extremely easy and others were way too hard for their CR. I could get annoyed at my DM a little for trying to kill us and not giving us enough XP to make up for it. But I couldn't complain that the DM was running the game wrong. It's not entirely his fault if he's just following the rules, right?
If AC 36 is too high and feels like a cheap shot unbeatable monster to your players it does not matter whether it is in 4e or 3e, whether it was created on the fly, came out of a book, or followed a formula.
It does. In the same way it matters whether you get 200 dollars for passing Go in Monopoly. If you don't get the $200, the game can still be played. It'll be a slightly different game, but the game doesn't suddenly break down because of a slight change in rules.
But you aren't playing the same game any longer. You can no longer claim that you are the best at Monopoly when you do really well with your new rules. You can't entirely be sure you'd be just as good with the real rules.
For me, it's always in the back of my mind that I may be winning...but would I be winning if the DM was playing fair using the rules exactly as written. If the DM decided 25 was a fair AC for a CR 10 creature and I look later and find quite a few examples of AC 28 CR 10 creatures....would we still have won if it had AC 28?
I suppose it's because I trust game designers more than I do DMs. I know a DM will VERY likely make a mistake when "estimating". I've seen it over and over again. DMs giving powerful artifacts that would unbalance a level 20 group to 1st level characters while claiming that it will have no effect on their game at all. Are these the people I was deciding randomly what the AC of the creature is? At least if they are following the rules in the book, I know that someone who knew what they were doing sat down and thought about those rules for a LONG time and came up with the "best" answer they could come up with. If a DM follows those rules, they should succeed no matter how smart they are.
It's just that I figure game designers had to have a job interview, qualifications, a team of people checking their work, playtests and so on. The DM running my game could sniff glue in his spare time for all I know. Most of the time, DMs get the job because no one else wants it.