You must factor in the time you spend learning to use your pressure washer, as well as all the time you take cleaning your home, or whatever else you want to clean off. All this time could have been spent doing something else instead.
By hiring a professional pressure washer, you save yourself a significant amount of time. The service provider can get the work done far faster than you could, so you’ll pay for a fraction of the time and effort that you would have had to put in. If you value your time, it can be worth it to hire a professional, even if you don’t factor in the cost savings.