There are a few reasons:

• The companies that most need devops / cloud help are startups, but unfortunately startups generally don’t have the money, infrastructure, or head count for a dedicated devops / cloud person.  What they generally do is look for a developer who either knows something about infrastructure or can figure it out, and commit a portion of that person’s time to sysadmin responsibilities.  There are a few problems with this, but the biggest is the opportunity cost: every hour that your developer is spending on infrastructure is a lost hour of development.  Also, as anyone who has tried to do this will tell you, actually finding somebody with that skill set is a lot easier said than done.
  
• Designing and building a scalable infrastructure is a lot of work, but the amount of work required for maintenance is comparatively small .  That means that you may spend a chunk of money in the beginning to get it done, but after that you can either pay by the hour for maintenance or DIY.  Either way, it ends up being a lot cheaper in the long run than hiring someone capable of building it out.
• Hiring a consultant means that you can get started on a requirements gathering tomorrow and have someone working on it next week, rather than waiting for the amount of time it takes to hire someone.
• Hiring a consultant doesn’t preclude hiring someone full-time – you can also hire a consultant just to keep you going until you find the right fit for a full-time position.