One answer to that, from a previous company was that they would far rather everyone take plog fuel (assuming no glaringly obvious reason to take more) on every flight, and suffer the occasional divert because things didn't work out, than have everyone taking extra just in case, which statistically isn't needed and have the extra burn.
Now I'm not suggesting that you just blindly follow what the company says, but at the end of the day, in theory, it's their rules and their choice how to play the game. If they would rather increase the risk of diverting that is up to them as long as it doesn't incur safety issues.
It's a bit like the argument of speeding up when running late to try to 'fix' the schedule. If the company would rather suffer poor time keeping and dissatisfied pax then that is their call.
Understand that I am not saying we take plog fuel when the destination is on limits and all alternates are iffy and likely to fill up with other diverts. That would be silly.