It's because it does not require an attack roll. Toll the Dead is not an attack either.Wow.
Not sure why it's not an attack, but that is a distinction Crawford makes.
That said, although supported by the rules, DM's are not always going to let it fly, nor will all players (including me) be ready to exploit what is basically a loophole to let a familiar take offensive actions in combat.
As for the value:
1) The big draw here is that the familiar can use it's action to use the spell, but keep in mind we are talking about a small cone of fairly low damage
2) The Wizard will need to maintain concentration on this spell. That's a fair cost for a fairly small attack 1/round.
3) The familiar is squishy as hell. This is not a hard spell to remove - one magic missile for example.
4) Also, there is the initial action to set this up
So overall, I think it's OK. Saving that action every round is good, but the effect is OK at best. Spending concentration on this is a significant factor, as is the ease in killing the familiar. Let's also keep in mind that the familiar's action wasn't useless before, so this isn't being activated for free, we are still using up an action to do it that could have been used for other things.
Mainly though, I probably would avoid the combo simply because a lot of DM's are going to either assume it's illegal, or think it's an exploit of the rules. I tend to avoid things like that, there's enough amazing things I can do.