You may well pay more in aerodynamic penalty on the bike trailer than you would in extra weight on the bike itself, though. It merits calculation.
Also ... biking with a trailer just isn't fun. I'd much rather bike on a loaded-down heavy bike than on a lighter bike dragging a trailer behind it.
And there's the fact that I'm sure you must be underestimating something, because a bike trailer has additional weight from its frame, wheel, and tire to add on that a fully-loaded bike does not. I don't see how it's possible that, for equal carrying capacity, adding a trailer could somehow reduce total mass.
I don't bike particularly fast, so I'll bring up other factors.
The wheels of the bike trailer add additional rolling resistance, and more I think than putting the same weight on two wheels. That is, the trailer wheels are smaller, which means there's more force against the direction of travel.
Most bike trailers have two wheels side-by-side. If you are biking down a dirt road, then odds are there are packed ruts side-by-side, with a grassy strip in the middle. With a 2-wheel bike, it's easy to go along one of the ruts. With side-by-side wheels it's more likely that one of the wheels will be in the grass.
(My spouse uses a trike, and has this problem.)
I think username223 doesn't understand where the weight comes in, and is estimating what the weight "should" be based on a road bike + trailer combo.
A bike like mine, for bike camping, is heavier than a road bike. I also use it for dirt roads, so it needs to handle more shocks and vibration. The racks and fenders, and of course the gear, adds more weight. There are also not one but two kickstands on mine - the one on the front tire is needed if I'm carrying front panniers, because otherwise the wheel may twist and the weight pulls it down.
This then calls for a stronger frame, which adds even more weight.
Also ... biking with a trailer just isn't fun. I'd much rather bike on a loaded-down heavy bike than on a lighter bike dragging a trailer behind it.
And there's the fact that I'm sure you must be underestimating something, because a bike trailer has additional weight from its frame, wheel, and tire to add on that a fully-loaded bike does not. I don't see how it's possible that, for equal carrying capacity, adding a trailer could somehow reduce total mass.