They can. The servers have settings for all kinds of paramaters. Bonus experience campaign is simply tweaking the value for exp gain to be double the normal value, so instead of 1x it's 2x. They could make it 3 or 4 or 5x if they wanted. Every bonus campaign is just them adjusting a single value on the server end to multiply the base value until the campaign is over, at which point they reset it back to the default 1.0.
There's been plenty of instances where people who host private servers have modified those settings so that you natively run at double normal speed, get insane skillup rates, and get far more exp or w/e than normal. If private servers can do it, so can public. They just don't because reasons. Maybe fairness, or equality, or balance or w/e. Or maybe they just don't want to. But if they chose to they absolutely could give smaller servers those kinds of bonuses fulltime.
Edit: I found a youtube video of some guy showing the configurations available when setting up a private server. Everything from default tp gain you get from hitting mobs to default run speed, skill gains to the amount of gil dropped when mobs die, exp loss when players die....etc etc etc. It's all in there. If you pause and zoom in as he scrolls down through the file you can see there's LOADS of options, and its all server specific, so they could easily modify the values for any given server to be different. They just choose not to.
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