It seems to me that the service interval for bearing on a trailer should be compared not to the service interval of a modern, sealed car bearing, but rather to the service interval of the front wheel bearings of a 1950's or 1960's drum brake car. Todays axles and brakes really are straight from that era.
On cars of that vintage, I used to re-pack bearings at every brake job. IIRC, that was somewhere around 20k miles.
I'd also consider the relative weight and abuse to which trailer bearings are subject. My 1971 Plymouth didn't weight 5 tons and didn't get dragged sideways around corners.