They often are over time. The advantage of a corporate rate is that it tends to be consistent throughout the year, regardless of how heavily booked the hotel is and seasonality.

From a corporate perspective, this certainty helps with budgeting and means that a reasonable deal is always achievable. I moan when I have to book our $130/night corporate rate when web specials are $69, but on the other hand, that $130/night rate certainly looks attractive during a special event when all the hotel rooms are going for the full rack rate of $350.