There are multiple reasons, but I'll give you the biggest reasons an underwriter wants to see your bank statements:
1.) To make sure the down payment is coming from an "allowable" source. For instance, if it's a gifted down payment, that may be allowed but SOURCING the gift from an allowable donor is required. One example, you may not be able to receive gifts from non-family donors.
2.) To make sure you didn't open any new loan for a down payment. Many loan guidelines actually will allow you to take a loan for a down payment (in fact, most of the Down Payment Assistance programs offered throught he Washington Housing Finance Commission IS a small loan for a down payment- Reference http://www.wshfc.org
). However, if you do take a loan for a down payment, the terms, balance and payment of your loan must be calculated in your debt ratio.
3.) To see if the money is coming from an account that is shared jointly with someone else. If it is shared jointly, you must get written consent that the funds from that account can be used for a down payment from the other account holder.
4.) (this is the major point Ardell mentioned) making sure you have enough funds to close. It costs time, energy and money to get all the way to closing and NOT having enough money to close. It seems uncommon and is definitely uncommon from seasoned loan officers, but it does happen from time to time and probably more often than we'd all like to believe.
There are many other reasons, but those are the biggest ones. I tried to cover these topics in a blog post about making sure your earnest deposit can be sourced.
The same reasons apply.
Remember that mortgages (at least nowadays) are more balance sheet lending than credit driven lending. You can get a mortgage with pretty average to below average scores if you have money down, good work history and the purchase makes sense compared to your expenses (including your new mortgage). Are they looking for large deposits and overdrafts? Yes, they probably are. However, they're looking for ANYTHING they feel should be clarified. That said, the 4 reasons above are often the biggest reasons an underwriter wants to see your statements.
FYI, if you're already having problems reaching your loan officer, you should consider switching. Being this is your first home purchase, you'll never really understand how important it is to have somebody who's available. They don't have to be local, they just have to be there when you need them in some capacity. Especially to answer questions like this. There are SO many other questions you may NOT ask because you feel they're too busy for you. You can keep asking in this community, which is helpful, but you want a professional and an expert working for you too, not just guiding you through from online answers on a forum. I know for a FACT that there are multiple people on this thread who would do a better job. I'd be happy to remove my name from the hat as an available loan officer to help you just to express how important this is (and how much faith I have in some people in this community). Your first home buying experience is so heavily effected by the quality of loan officer and lending institution. For every first time buyer who has a great experience, there's often another (if not two) other buyers who had a horrible experience. If you want to see for yourself, start a new thread called, "Real Estate Agents, how many transactions did you have fail due to financing in the last year?". Trust me, it won't be a pretty thread.
@Ardell and Rhonda, It's good to see familiar faces here! Looking forward to seeing you two at the next barcamp.