I'm not sure where modern smartwatches stand in terms of accurate readings but if I remember correctly it is still a minefield of inaccuracies. A watch for 50 dollars will _probably_ not do as well at reading your vitals as a watch for 200-500 dollars. Given Whoop markets itself as backed by science and seems to take the lead in studies on accurate heart rate readings[1].
Couple the above with great marketing and a decent kit of software and you have a difficult champion to dethrone. Hardware is expensive, marketing is difficult, a good app is hard to make, and putting together all three at once _while_ fighting a niche market is a struggle!
While 30/month can be difficult to swallow for someone that runs or swims as a casual hobby, someone that already pays hundreds a month for equipment, coaching, gym memberships, can have an easier time justifying it.