I have no issue with banks failing for these reasons. In my opinion, a bank's failure resulting from overly risky activities is the desired outcome of a capitalistic economy. Nevertheless, I believe we share the same view on this matter - that regulation is warranted - only for different reasons. Specifically, my problem with the failure of these banks is that the FDIC insures their deposits. Because of this federal insurance, consumers do not have nearly the same incentive to be informed with respect to a bank's lending practices. They don't necessarily care what happens to their money, simply because the federal government backs it. When was the last time you walked into a bank and asked, "what types of actions do you do with the money that I lend you" or "what are your minimum reserves". Fankly, that isn't something that crosses the minds of bank customers anymore [although it did in the past]. Not saying I don't find comfort in my money being insured, but it distorts the incentives for the banking customer and has caused the need for more intervention. If there were no FDIC deposits, customers would be extra careful to what bank they loaned their money - likely only to banks with insurance on deposits and those that had significant internal restrictions on the use of deposited funds. Nevertheless, that isn't the situation we are in today and I see only two solutions for this problem: get rid of the FDIC (which would be a political nightmare because there is presently not much of a market for private deposit insurance) or regulate banks' uses of FDIC insured money.
As to your question about why we don't have legislatures running around screaming market failure all the time is fairly self-evident in my opinion. Market failure exists where the market produces a result that is not the most efficient possible. Most commonly, examples of market failure are non-competitive markets (such as monopolies), externalities (such as pollution), and public goods (such as national defense). According to many economists, market failures are the only reason to intervene in the market. The reason, I believe, that you don't hear legislatures running around screaming "market failure" for each and every project they choose to undertake is because - quite frankly - they have no idea what "market failure" even means (much less how to spot a true externality or public good). By introducing the topic into debate, they would open themselves up to a significant amount of criticism as much of our federal appropriations are not in response to any sort of market failure.