Bacon Bits
Legend
This seems odd, you will hit and be hit more and more as you advance.
Yes, that's by design. Conceptually, you actually get better at attacking as you gain levels. Your character actually improves and gets better at fighting, and is better able to cause their opponents to tire out and stop being able to defend themselves. Similarly, since your hp improve, you're better able to fight other characters who themselves are better at fighting. You don't survive because you have high AC. You survive because you can hit them more consistently and you have the hp to endure their attacks.
The alternative model is "being stuck on a treadmill" where you start the game hitting 50% of the time at low level and end up hitting 50% of the time at high level and do marginally more damage even though hps are much higher. That was one of the complaints with 4e, whether you agree with it or not. This system does scale indefinitely, but it's a false progression because you never actually get any better at doing anything. You're just adding bigger numbers to have bigger numbers. It's also very difficult to make lower level opponents be threatening to high level opponents, regardless of their numbers.
Also seems odd that the goblins we first encountered had a higher AC than most of us adventurers (15).
That's kind of unusual. Any class that has heavy armor proficiency can begin the game with AC 18 easily (chain mail and shield), and any class that begins the game with medium armor can begin the game with AC 16 easily (scale mail and shield or +2 Dex). If you've all got AC below 15, you've got a very unbalanced party as far as equipment draw or your players are undervaluing Dex and shields.
AC is almost always a factor of your wealth, not your level. if you're a Fighter, once you get 1,500 gp and buy into full plate your AC is basically set for the whole game. You might find +1 armor or a +1 shield, but you don't have to. AC just gets less important as the game progresses; at high level, hp is where it's at. There are several monsters in the game with AC 17 and higher as early as CR 3. Animated Armor is CR 1 and AC 18.