It's not just banks . Many companies have sold off real estate over the years as they see it as capital tied up that could be used elsewhere . Another example is ' just in time ' manufacturing that most of the majors use . No sense having all that capital tied up when it can be used for a higher return somewhere else . Most management people these days have short term goals and effectively work from year to year budget wise . If they can see a short term gain ( free up cash ) then they'll surely take it .
Think about the value of property that banks own . Centre of town , prime position etc . That's some blue chip RE that they are sitting on .
Also , as banking services change to online and phone , having that physical structure is probably not needed any more . I imagine that you may not be able to walk in to a bank ( in it's current traditional form ) in 5 years time .
