I'd argue that complexity is what causes loopholes, rather than the other way around.
Things like tax credits aren't 'loopholes' - they're specifically codified in the law for people to utilize, in an attempt to encourage certain behaviors.
It's not gamesmanship, although there is some competitiveness between state tax laws (such as this WA vs CA capital gains thing).
That's WA's incentive to get people with lots of money to come live (and hopefully spend) it in WA. CA has enough people with lots of money that it can afford to take a different stance.
That's a pretty bad attitude to have, that citizens should be trying to exploit as many loopholes as possible. That sets up citizenry vs. govt. as a competitive game, which is a great recipe for making something that's supposed to be cooperative become dysfunctional and complex.
It's an impossibly complex task to make a perfect airtight literal codex of laws in a system, and it's not reasonable to assume that humans can create such without introducing a huge number of contradictions, etc. We need to keep in mind the spirit of the law in order to keep our system from devolving into a morass.