The reason, for me, is almost entirely pragmatic. Copyright and licensing are protections, but in today's world, receiving the benefit of that protection requires a budget, a lawyer, and a willingness to undertake legal proceedings which could take years to resolve.
If I am willing to invest in (1) registering my copyrights, which in the US is the only way to receive damages, (2) ongoing and perpetual searches to make sure no-one has violated my copyrights or licensing, (3) the costs to have one or more lawyers write letters, sue, and defend the suit in court against a likely large corporation who will spend millions to delay the case and bankrupt me, then I absolutely should put a license on my work and actively engage with my copyright.
If I'm not, regardless of whether I cannot afford to or do not desire to make a lawsuit my life's work, then I should opt out of the entire system. That is usually my choice.
( As a further note, Yale Law professor Stephen Carter argues that there's a fourth condition there: (4) if I'm willing to kill someone to enforce a legal verdict in my favor. See the quote in https://www.theatlantic.com/politics/archive/2016/06/enforci... )