One of the reasons spell is weak is...

...the supports.

Think about it, attack supports are better than spell supports. I'll just use lightning as an example here, but it can be translated to other elements.

First of all, attacks get Elemental Armament, which is just a flat 25% more damage. No downsides at all. Then they get Attunement, which lowers other elements, and 90% of the cases, it is not a dealbreaker since you'll be using the chosen element anyway. So now you get 25% damage as extra damage. Lastly you get the 5 seconds buff on kill from Innervate (or Ice Bite, seems like they forgot to add a fire version of this gem btw!)

So that's pretty much 75% damage for -free-, basically no downsides at all. Now lets take a look at casters.

All spell supports have a tradeoff, and they are often awful.

To start with, you have no elemental specific damage increase like elemental armament, or attunement. So you can can pick between controlled destruction, which is 25% but completely eliminate your chance to critical strike or considered casting, which is a 35% but gives you less 15% cast speed. Which effectively makes it a 20% buff.

There, those are the two spell ones. Both with massive downsides to a caster. You can get deliberation of course, but so can deadeyes, and they can also have heralds to back up their damage while a caster cannot.