eQGy18 is right. Also, those consulting partners have done a gazillion implementations across many verticals and have a ton of diversified experience. You're paying for the expertise of every company they've ever worked with.
Because they increase your reach in a way that would take even a fast growth company years to scale up to.
also, they are very cost effective and people companies Don't want to buy white elephants where buying them is x USD and if the company installs it is 2x to 5x. instead get a partner who can do similar stuff at much cheaper price
Because most software companies don't have implementation people and partner with SIs to implement their solutions. Implementation is low margin business and will drag down the software company's margin and valuation of the business.