If the jack has its jump, then 4 glaives do no such thing (unless you mean 15secs or so assuming they are in position and the jack has no jump available). 15 seconds is hardly moments when a jack is pounding away. Further, if the glaive player makes 1 minor micro error, then the jack will 1 shot a glaive. Guess how much damage a jack can do in 15 secs -- 4500 - that's enough to kill all factories, except the ship factory. If 1 of those glaives ventures too close and dies to a jack, then that damage is even higher.
Add the mobility of a charon and a jack becomes a better raider than 700 metal worth of any actual raider. Raiders also must survive across the entire map (and usually be micro'd) to raid. A charon can easily be given waypoints, a drop command, and return waypoints, in queue to save all the micro time plus probably save the charon too.
The principal reason jack drops work is the low cost in relation to hitpoints, mobility, and dps. Its short range is meaningless against economic structures that cannot move. Its 6k hitpoints mean that without a 9 gnat swarm handy to counter them, an early jack will easily make more than its cost in a raid. There is also the stupid mechanic that jacks can actually hit gnats that are trying to stun it - jacks need not even jump to do so. A 9 gnat swarm will almost always lose gnats against even a single jack because jacks 2 shot gnats. Not to mention that a 9 gnat swarm costs 810 metal v. the 700 metal of a jack + charon.
The other option is to shoot down the charon with swifts, but you'd better have an air player with 4-5 swifts ready + that player must react in about 1-2 seconds or the jack will still make cost. The charon will die too, but that's hardly a consolation to the player that lost a factory.
I know jacks are usually only a problem in lobpot, where the 2nd factory for gunships easily is viable, but that makes it no less toxic. Depending on how early it's done, a jack drop factory kill can easily win an entire lane. In lopbot, I've seen it done inside ~1.5mins (the actual match is in another thread).
There is NO cheap easy counter to a jack drop like the lotus is to most early raids.
