And how long did those horses last as useful, working animals? A riding school, for example, that treated their horses like horses were treated in e.g. WWI would be quickly shut down!
If you think in terms of repeat business, having one horse that can carry, say 18st, isn't really enough for a riding school to advertise a weight limit of 18st, because what if that one horse goes lame and you have to turn them away? You also don't really want a horse working right at its weight limit all the time. So, we had a stated weight limit online, but we were flexible with it in reality- and we still had people we needed to turn away due to weight.
And it's true for most businesses, that you don't want to work your plant and machinery or your staff at anything near to 100% of capacity for long periods of time.