Two reasons. first, there is no real evidence that dragons have any degree of actual immunity to human magic. Second, it is too much. Dragons have consistently shown power immensely greater than that of humans. In terms of size laxus did use pretty big attacks but I doubt they come close to what dragons have shown so far. Take a look at the roar mothergrea used. There is no way what laxus used against ajir even compares to that. Sure, the scale of the attacks is considerable however motherglare's roar traveled past the city well into the distance and caused a explosion which seemed huge. In turn what we saw from laxus was pretty big but also in the immediate vicinity. Laxus at this point might be able to defeat a dragon. However him actually matching the raw power of one probably won't be a thing.
To put dragon's power in context... We just saw natsu almost killing zeref. And what did natsu used? Leftovers from igneel's power. Just how immense was igneel's power that leftovers from it were enough to work? Ok, maybe it is not outright impossible for a human to match a dragon in terms of raw power. However the implication so far is that dragons have a volume of power so absurdly immense that even a punch from makarov is nothing to one much weaker than acknologia. It is so absurdly immense that even when faced with a dragon slayer of laxus caliber 2 years ago zirconis walked out of it pretty much
unscathed. And there is no reason to think dragon slayer magic is less effective against dragons than it is against demons. How many times stronger than makarov do you have to be to not even flinch at his attack? dozens? Hundreds? thousands? Maybe it is plausible for a human to eventually acquire that sort of power... but over how long? I doubt even among the sprigan there are people who have dozens of times the power makarov has. And to be fair, zeref gathered the springan to defeat acknologia.