Two reasons: IMO primarily because it’s written to address people who are learning about the market and likely in the accumulation phase of their financial life. They are likely to have money to invest at regular intervals (and if they don’t have money to invest, knowing when/how they should is fairly academic).

Secondarily, if you say won a large cash lottery, you might choose to invest that over a series of spaced investments to ensure you got some kind of blended/average price rather than facing the market-timing dilemma of “should I invest this on Monday or wait until Friday?”

How is that not timing the market?

The premise of “time in the market > timing the market” is that all you know is that at a point sufficiently far in the future, the value will be higher.

Which means at any given point in time, if you have the ability to invest, and your goal is to achieve long term returns with low risk, then you simply invest as soon as you can. Since the longer the investment is held, the less volatile it should be. You are not trying to maximize returns, you are trying to minimize volatility.

Timing the calendar is different from timing the market. I internalize the latter as “buying when you think the price is lower than it should be and/or selling when you think it’s higher than it should be”.

(I agree with your approach and do not make an effort to dollar-cost-average or otherwise smooth my investments over time. But I don’t think of a pre-defined calendar based approach as “timing the market”.)