Question: When looking for consolidated storage, why shouldn't I just build my own system instead of buying from one of the storage array venders?
Answer: Frankly there isn't any reason why you shouldn't build your own SAN if that's what you have the budget for. However if you build your own system using open filer or Windows Storage Server you'll have a lot of extra work that needs to be done to get the same level of enterprise level support that you would normally get from a storage array vender.
Storage array venders like EMC, IBM, NetApp, etc. spend billions of dollars designing and performance tuning their software and hardware. When you build your own platform you don't get this sort of performance tuning. You have to do all this work yourself to ensure that you get the best possible performance. Additionally when there is a performance or configuration problem you don't get access to the resources that a vendor would normally make available to you such as their support team and their performance support team which when using a vendor supplied array you get as part of the purchase.
For smaller systems and solutions platforms like Open Filer and Windows Storage Server can work just fine as native systems. When it comes to larger more complex solutions purchased solutions are usually better so that the sysadmin and storage teams can focus on managing and tuning the business systems and not focus on building the storage array from scratch.