Right, but a large percentage of people selling SEO don't actually know how to rank pages. That's just the reality of the industry. Even the best SEO takes time to rank pages, that's a fact so a buyer doesn't have any choice but to buy SEO from someone. If a person knew SEO there's a good chance they wouldn't be buying SEO to begin with.
Then you have folks selling SEO that mislead noobs/buyers into silly schemes, example, anywhere on page one in the SERPs is great (wrong) position #9 sucks just like position #49. Now there's a way to turn position #9 into outranking position #1 with Featured Snippets for some keywords but most $5 SEOs aren't selling stuff like that because it's a bit more advanced than push button nofollow link blast that don't require any skills.
Also keep in mind, a lot of buyers bring on there own problems when it comes to shopping for an SEO, a lot of of buyers want everything for nothing (ex: Fiverr). So it's not always a bad SEO at fault, there's a niche of buyers wanting something for nothing, so there's going to be sellers selling something for nothing, even If that something is useless.
It is what it is.
Like I always suggest, when buying SEO start with very small projects and work your way up to your ultimate goal. This way you won't be out a lot of money trying to find a decent person to do SEO. Same goes for anything else, even offline and local, plumber, HVAC person, mechanic, etc... don't trust anyone with your money until they prove they can be trusted.