This isn't quite accurate. Since 2007 there's been much more regulatory pressure--particularly on large commercial banks, but all financial institutions to a lesser extent.
A major issue is the regulations themselves are extraordinarily complicated and the regulatory regime is unclear. What you characterize as "deference" is more like this: the regulators don't know what regulations out of (literally) hundreds of thousands of pages' worth of regulation they can get away with enforcing or not enforcing, but for political reasons they might be called to enforce whatever regulations at whatever time, such as with the retroactive enforcing of mortgage regulations everyone was ignoring before 2007 because the political incentives were such that everyone should have access to mortgages. So regulators are incentivized--whether corrupt or not--to build soft, close relationships. (As an aside, regulators and banks are continually building sophisticated machine-learning algorithms that have access to all your financial data, as well as powerful search systems to find whatever a regulator might demand.)
Hobbes, writing in England in the 1600s, described the "silence of the law", wherein if the king's agents enforced all his laws his rule would quickly become tyrannical and lose legitimacy. Well, again, there are literally hundreds of thousands of pages of financial regulation, depending on who you ask, that can be characterized as "must follow". Enforcing them all would draw the wrath of most Republicans and moderates, because the world does need a financial system.
The regulators work softly because, honestly, they don't know what they are doing or are supposed to do. Banks try to get in bed with regulators because they don't want to end up like HSBC; people calling for their heads essentially because they were lazy about regulatory demands for an issue that ended up being politically sensitive. A large part of compliance nowadays is trying to predict what Congress cares about or might care about in the future. Regulators are glad to get the extra access. After all, when the political winds blow the right way, they have the real power. Otherwise, don't rock the boat.
There is also corruption--always, everywhere--but if I was to imagine how a "good" regulator could do his job in the way Michael Lewis demands, well, I can't imagine that ever happening.