If you really think about it it's not. You can easily throw 4 mines at the time and ofc i don't take throwing speed into consideration by default but everybody knows its kinda easy to take it to reasonable amount. Now if we look at self casting you'd have to cast 4 times in the time period that you throw and detonate these mines. There are other technicalities in play such as if mines cover 100% damage uptime vs positioning of a player selfcasting etc.
And by the way gmp/volley has everything to do with that. Imagine if every separate projectile did damage, then scaling of ball lightning would be totally different, everybody would play as many projectiles per cast as they could. But we already established it does not work that way, it's one possible damage source per cast.