Think for a moment about an "average" experience of coming of age in a modern developed nation.

You have been getting an education from the age of 5 to 22, give or take a couple years (and potentially with a gap for military service, in countries like Greece or Israel). Notice, in the middle ages, you probably would have come of age much earlier. Longer lifespans and more complex societies mean longer periods of perceived immaturity, among different human societies. Just imagine how much stronger this pressure would he for an elf. Even if they are biologically mature at 30, if they live to 800, they aren't likely to be considered full adults until decades or centuries beyond that point.

Back to our modern day example. You are likely in some form of debt to pay for this education, so you get a job to pay off this debt. If your country has a civil/military service requirement, you may have paid off some of this debt through service; perhaps this is applicable to Ellen societies. Either way... Sure, you have a college degree, but you have no experience, beyond maybe an internship, so you start off with an entry level position.

There are really two ways up the ladder, whether you stay in the same company or not. Either the economy is growing and creating new jobs at a higher level than yours, or someone above you retires, opening up a position that either you grab, or another higher up does, opening THAT position.

Let us imagine a future that, in some ways, has already begun. Same scenario so far, but the setting has changed. It is now a few hundred years from now, and Earth's population has been at a stable 15 billion for over a century now. While the economy still grows, it mostly does so through increases in automation, and the number of jobs remains constant as well. You come of age after 30 years of schooling and land an entry level job. The problem is, radical life extension technology means that people are living to 300 to 500 years (and by the time someone reaches 300 years old the technology tends to improve further, buying them more time). Sure, every once in a while someone in upper management is killed in a horrible accident that destroys the brain beyond even FutureRepair, but even then there are dozens of applicants with dozens of decades of managerial experience. Besides, no way a two hundred year old is gonna listen to YOU, even if you ARE technically their manager. Come back in a century, scrub.

Now let's apply this analogy to elves. Maybe their educational system is one of apprenticeship. Even if we have a mixed society, an Elf Smith isn't going to take on a human apprentice and squeeze 100 years of lessons into 10; meanwhile, no human will apply to apprentice for an elf if it means three decades of learning about proper forge maintenance and how to work the bellows before ever picking up a hammer. Elven crafts would be considered vastly superior to human crafts, but while a human dominated society might still see its king wield an elven sword, they would recognize that giving the army human forged swords is good enough. Not so in an Elf dominated society.

So let's say you are an elf coming of age in an elven society. You just mastered smiting after a century of apprenticeship. But guess what? You aren't a 'master smith' yet, not to other elves. You're an entry level smith. And your society doesn't just want elf-forged swords; they want your master's swords. Until he finally dies 500 years from now, nobody is going to listen to you talk about "innovation"; you're just an apprentice.

It must be incredibly tempting for at least some elves in this situation to say "screw this, I'm not waiting 500 years. I'm gonna go be a master Smith among the humans!". Of course, this would make elf dominated societies look down on them, and even if they return after 500 years with all they've learned among the humans, they'd never be considered a master Smith. For the majority of elves in elf dominated societies, perhaps this taboo is just too strong -- for much the same reason a human Smith doesn't move into a goblin village to practice his craft with no equal.