They get all kinds of "high profile" designers via just hiring them freelance for a particular product. The difference being... Wizards actually owns and makes money off the product the designer produces, unlike the OGL where it's whatever company that produces the product.
If having "high profile" designers and developers actually mattered to them... they would have eliminated the apparent contract legalese they have that states that anything a full-time employee creates on their off time is automatically owned by Wizards, even if its not D&D related. THAT'S why they don't have so-called "high-profile" people working for them full-time. It's why Monte left, it's why Stan! left, and why Bruce Cordell eventually left. They couldn't work on non D&D things while working on D&D because of the contract stipulations (which I don't know if we know are Wizards-mandated or Hasbro mandated.)
So instead, Wizards hires them on a freelance basis and everyone is happy.