Plenty of hosting companies offer managed hosting as an upsell. You can't even get pricing for it at Softlayer without talking to a sales rep, for example.
Rackspace is the only large hosting provider who provides that across the board, because even their smallest cloud servers come with phone support. As far as I know, you cannot get phone support with an arbitrary set of cloud servers at Softlayer, or Amazon, or Google.
No one is forcing anyone to buy from Rackspace. The question is whether Rackspace has a unique value proposition against Google or Amazon, and the answer is support.
Rackspace is the only large hosting provider who provides that across the board, because even their smallest cloud servers come with phone support. As far as I know, you cannot get phone support with an arbitrary set of cloud servers at Softlayer, or Amazon, or Google.