I have worked in these "consultancies" so here is my take. There is a wide range of talent in these consultancies. Some examples are web app developers/software engineers, website people, implementation engineers/enterprise app software, supply chain, cybersecurity, etc. Yes, unfortunately, some of this talent is not familiar with basic software tooling like basic linux commands or git or setting up a path variable. This sounds really bad until you work with a Georgia Tech computer engineering grad and realize he doesn't know this either.
Many methods of sales. A sale can be considered a billion dollar deal or having a director hiring a contractor. The billion dollar deal is closer to consulting. The director hiring a contractor is closer to professional services firm.
Prior to these consultancies, the orgs would hire their own employees and train them. What changed? Contractors easier to layoff than W-2 employees. Maybe their own employees didn't have the expertise.
The truth of the matter is for most organizations, IT is a cost center along with legal and finance. It is not their money maker. They have no interest in innovative IT/software "technology" until their competitor in the same industry as them has it. Price matters. Laying off people easily matter.
"Consultancies" exist for that reason. Look past their marketing/branding and their official classification is a professional services firm.