The OEM S21s that came with my '18 were well behaved for road riding. Mileage-wise, they were about average for sportbike tires. Mfrs don't always spec OEM tires based on cost alone. These carry the same speed/load rating as aftermarket replacement S21s and likely have the exact same carcass, if not very very similar. It actually costs more $$$ to retool/recalibrate the production line for a limited run of mfr-specified tires off the same S21 production queue. Tire compound might be tweaked a bit, which is a lot easier to do, since it's just pouring different mixes of pellets into the various bins. Contrary to popular belief and internet folklore, mfrs aren't always penny-pinching and skimping on OEM tire quality (though that does happen). Rather, they pick tires and compounds based on what their expectation of what would work best in that particular bike. These bikes have one chance of making a first impression, especially to moto-journalists jaded with having sampled potentially far better handling bikes. What those journals say about how the bike feels on the stock tires are often what consumers base their buying decisions on.
OTOH, new owners have widely varying expectations on mileage, handling, grip (both dry and wet), and more vain qualities like aesthetics, brand name, and bragging rights. A lot of these qualities are competing goals. It is inevitable for many owners to displike OEM tires, which by their very mission have to be generalists. Some think they don't last long enough; others badmouth them for not being high enough spec'ed for ultimate performance. This is particularly true for sport tourers, which again by definition are supposed to be jack of many trades and master of none. Heck, some just flat out don't like a particular brand, cus some guy with that tire brand screwed their girlfriend.
Besides, folks like to choose. Just look at how many "which tire is best" threads there are. With OEM tires, that choice wasn't theirs. Once they do get to choose, they will most likely be confirmation-biased toward the one they personally chose and paid $$$ for. Consumers studies have shown that there is a strong positive correlation between price willingly paid and perceived quality/performance, whether differences actually exist vs. less costly options. Who wants to admit they paid 2x more for something that only works 1.25x or no better?