Comparison with hosting your own email is like a dozen orders of magnitude off in complexity. SAP really only makes sense for ginormous corporations, but up in that range, Oracle's ERP is the only real competition. Just the kernel of SAP is on the order of 100 million lines of code, and that's not counting the even more hundreds of millions of lines of business logic.
It'd be more like saying, "I'm going to reinvent email, write all clients, servers, and try to get everyone to adopt it." Except that would still be a few orders of magnitude off.
SAP has modules that handle regulatory issues in virtually every country in the world. There's so much business complexity that's encoded into its systems that it's literally hard for mere mortals to grasp.