Here you have a buck conveter where Vout1 = D*Vin which is positive for positive input voltage.
I'm wondering why don't we just reverse the connection of output like Vout2so we can get a negative buck output instead of using inverting buck-boost converter to generate negative output?
You're on the right track for one fairly common method of generating a negative output voltage. Take a look at the circuit below:
simulate this circuit – Schematic created using CircuitLab
The main downside to this is that the controller is no longer ground-referenced, so unless it's designed to be used this way and has level-shifters built in, you'll need to level shift any signals that go between it and the rest of the system. Another downside is that the controller needs to be rated for Vin-Vout (a higher voltage, since Vout is negative) instead of just Vin. But if you can get past those hurdles, it's an effective way of generating a negative supply from a positive one.
Note the difference between this circuit and yours: in this circuit, the control IC and the low-side switch are no longer connected to ground, but to the output instead.
Your idea is correct, in that the polarity of the output of a single supply just depends on which one you "call positive". You could connect any component or circuit to the output to get one polarity, and simply flip the output connections to get the opposite polarity.
Problems arise when you try to combine buck converters (or any type of power supplies), to obtain dual, or multiple voltages, some positive, some negative.
Start by noticing that most DC-DC converters, like the one you show us, have a common connection between input and output. Usually it's called ground, or 0V, or "negative side". Your own circuit shows this as a wire connection directly from input-side ground to the output side.
You might think you could power two buck converters from the same source, and connect their outputs in series, to form a dual "+5V, 0V, -5V" supply, but what you would actually be doing is this:
simulate this circuit – Schematic created using CircuitLab
Following the blue path, you can see that I've accidentally short-circuited the output of the lower buck converter.
When combining supplies in series to obtain a negative voltage with respect to some common ground, you run into problems if the supplies' outputs aren't completely isolated (on both positive and negative sides) from the inputs.
The designer of a negative DC-DC converter might solve this problem by ensuring that it is the positive side that is common to both input and output. In that way, when combined with a normal buck converter (by connecting their outputs in series), that output short-circuit doesn't occur:
Perhaps the most important thing to notice in this last diagram, is that the lower converter produces a -5V output when provided with a +12V input. This implies that its output is not merely a reduction in voltage magnitude. If you took a voltmeter to measure the difference between the battery positive and the -5V output, you would measure 17V difference!
The change in sign is not just cosmetic. The negative DC-DC converter is producing a voltage difference which is not just 12V to 5V, it's +12V to -5V, a difference of 17V. It allows designers to declare a common ground, and produce voltages below that 0V level, even when the source is greater than zero volts.
