why are there no attack trigger gems?

There sort of are, with ballistas and traps and mines. EDIT: well, really only ballistas

I think by their very nature, attacks can't be "triggered" in the same way spells can, because attacks require the weapon to work. If, say, a bow attack was triggered by a spell away from you holding the bow....how would that work?

BUT

Using a mine/ballista/totem/trap that uses the skill itself, or otherwise recreates the weapon that uses the skill does make sense.

What would be the point for triggering attacks from spells?

The reason triggering spells is so effective is because cast speed is by default much slower, and is also a lot harder to scale than attack speed.

It also used to be a way to bypass mana cost, but that was removed since 3.15

It exists. Look for Replica Maloney's Mechanism.

Well there is manaforged arrows too. But thats another attack to attack

Counterattacks just make sense: they HAVE to be triggered otherwise they wouldn't be counterattacks. I'll let YOU try to manually use a counterattack...Mirage Archer is somewhat more of a minion-like skill than a trigger skill, especially if you detach them with ascendancy so that isn't really a good example either.

A "trigger" requires something to happen automatically and it doesn't really make sense, unless under very specific circumstances, for an ATTACK to be the thing being triggered, given that an "attack" literally has your body moving in a certain way. Spells are mostly made out of thin air or otherwise don't particularly have any physical connection to your body or your gear. And so they are much more easily automatically created as you are currently doing other things.

Attack to Attack triggers make sense, because you are already performing the same action as the trigger: attacking with a bow triggers other bow attacks because you aren't conjuring some completely separate bow, hand, body to do it.

Can you imagine if you were casting fireball from your hands, and that forces you to suddenly jerk out your arm and fire a projectile from your wand? That's just a really awkward, nigh impossible (yes I know this is a game...), thing to happen.

BUT what they can do is create a Replica Maloney's-style unique wand that does exactly what you describe.

Replica Poet's Pen perhaps?

I'm saying it makes sense for an attack to trigger another attack because it is the SAME action. It would be like instead of firing 1 lightning arrow when you notch your bow, you fire 1 lightning arrow then 1 fire arrow in quick superhuman succession. There's no separate action really taking place.

A spell triggering an attack on the other hand would trigger the CHARACTER to move in a certain way. The spell doesn't come out of the weapon itself, it comes out of the air. Your hands, arms, whatever can be in a completely different position from what would be required for an ATTACK. Therefore, if the spell were to trigger an attack, it would force your body to move in a certain way. It is NOT the same kind of action.

Attacks triggering spells makes sense because the spell could be like an enchantment on the attack, that gets cast or released upon detonation or connection of the attack. The body wouldn't necessarily have to do any extra movement in that case: the spell just happens out of thin air. Attacks can't work that way, that is the very essence of what makes an attack different from a spell.

All I'm doing is injecting a bit of logic that MIGHT be behind the decision to not have spell-triggered attacks. Similar to why minions cannot trigger spells, or certain things like totems cannot be triggered. It physically doesn't make sense for them to be triggered.