In simple terms: because it is a tool everyone uses and knows to some degree. It has all the bells and whistles to do pretty much what you want with it. And, I guess one overlooked fact: the data is portable, everyone* in the world has Excel.
My view is that if you switch companies, and at the last you used SAP and the new one you have to use Oracle, it is going to be a learning curve with very little added value for you (except the fact that now you know to proprietary systems). With Excel, well, it is going to be the same Excel, just different data.
The fact that it's already installed on almost everyone's computer in any given corporate environment also has a lot to do with why it's so widely used for these types of things I think - noone needs to ask permission to get a new app installed, or jump through hoops to pay for another new piece of software.
My view is that if you switch companies, and at the last you used SAP and the new one you have to use Oracle, it is going to be a learning curve with very little added value for you (except the fact that now you know to proprietary systems). With Excel, well, it is going to be the same Excel, just different data.