Kitteh asked: "Why do professions not level up at the same rate as weapon skills? I mean, weapon skills go up 5 at a time per level, up to a cap of 400 at level 80.

But, once we reach Hero Online Gold master professions, we have to level 7.5 skills up at a time if we want to reach the cap of 450."

There's history to how the two skills got to be how they are, in terms of changes with patches and expansions. But that's kind of a side-step to the question. What you asked is "Why do professions not work the same way as combat skills?"

The answer is that they're different systems. Combat skills function as a variable inside an equation and become meaningful when functioning inside that equation compared to your opponent's combat skill.

By comparison, profession skills directly represent how advanced your character has become in a craft, and what recipes you can know.

The best answer here is "Professions and combat skills level differently because they represent much different aspects of the game."

You actually nailed it, there, Rainstorm. The reason forsaken and death knights aren't the same, treated the same, or have the same physiology is that they're different. If an undead and forsaken were the same, then an undead couldn't become a death knight.