That's selection bias, not all prostitutes hate their clients. Prostitutes that do would use that sub-Reddit to vent but I have actually managed to overhear some prostitutes talk to each other while going to the toilet in brothels and by eavesdropping in while taking by bicycle. The ones I heard talked quite respectfully over their clients and I don't think that they all hate their job.
A slut friend of mine said that as a girl she wanted to grow up to be a prostitute as her parents were bartenders at a brothel. She grew up to give it away for free to men, though by the time I met her she was married and her slutty days were behind her.
Prostitution is like any other service job, most that do it don't hate their clients but the toxic ones that probably don't know how to actually service a client go online and complain. I know quite a lot of people who work in the service industry and loves their job but if you go online all you ever see is people just complain about their clients, they might even talk about "customers" in the abstract and just mean one or two specific people.