wonder why frag cannons have so much more dps than all other weapons?
Firstly because (subject to @kittykatze's point below] the DPS is wildly inaccurate in Outfitting, to the point that from inception my damage tables including a warning about it [
see Kinetic spoiler]:
https://forums.frontier.co.uk/showthread.php/246086-Official-FDev-Damage-Stats-for-Every-Weapon
Secondly because Frags are one of the worst weapons in the entire game against every target except a slow, large, unsuspecting one. Although, of course, the game does provide slow, large targets, and all NPC's (/and some players) are unsuspecting...
I use the wing assassination missions as DPS testing ground for weapons

There frags totally outclass other weapons.
Yes indeed. Frags are your perfect choice. Unless you can get lucky with RC mines it impossible to do higher DPS against the slow, large, unsuspecting target that is a wing assassination mission principal. Furthermore here the game is finally letting us outfit for a PvE fight as we would for a PvP fight, i.e. for one engagement only before returning to base (unless stacking these missions).
The in-game DPS is wrong. In-game DPS does not consider reload times- it's just a simple [damage]*[RoF] number. As you likely know, frags have an extremely high DPS, but a small magazine and a long reload. Since reload is the main thing holding them back, not including reload in the DPS throws the numbers waaaaay off.
G5 Rapid Fire + Screening Shell reduces reload time to almost nothing and results in ridicoulous sustained DPS numbers very similar to that original displayed number :3
Example here.
The other day I was trying to come up with an in-universe excuse for why they're so powerful so that I could justify them RP-wise. Since even the notion of an anti-ship shotgun doesn't really make sense given that its armored plating and not flesh... I got nothing. Does anyone else have any ideas? Maybe if we imagine it as something completely different than what it is?
Frags were adjusted in 2.2.03 to have relatively low APV, in order to be more shotgun-ish, so actually I think the shotgun idea works. They are very dangerous against shields. (Albeit ofc corrosive makes up for the low APV as required - but that's another issue.)
Concerning rapid fire plus screening shell whilst you are correct that reduction in reload time to less than one second cures the DPS calculation issue that I and Frenotx identify, in terms of actual in-game use, Frags are used momentarily during passing / joust pursuit ... unless the target is large, slow and unsuspecting. In the latter case frags can be fired continuously, particularly if the attacker is lucky enough to 'pin' the target by getting one's nose stuck into their shield bubble. In this example, rapid fire screening can do full DPS, obliterating a shield and a ship.
But otherwise, medium v medium or small v small, frags are one of the worst duelling weapons in the game due to their limited usage requirements.
Admittedly, it is possible to overcome the range issue by using Pacifiers. But even with great stats, a fixed-only weapon with lower than 700 projective velocity and a range limit of 2 km is always going to lose in serious PvP.
In PvE frags illustrate the game's current failure to provide responsive targets. AI could (and I hope one day will, at appropriate levels) neutralise frags as easily as players can.
Don't get me wrong everyone, I love frags to the extent that I did a PvP video series about a character called '
Fraggy McFragface' ... but I recognise the limitations of this weapon as well as its benefits.
Here's the vid of Fraggy's final outing. The point is that in most of those fights most of my opponents (even though many were experienced PvP-ers) did not know how to fight frags, hence were annihilated. But that was two years ago and now, most do know.