Well.............I can't say that your wrong. LOL!! Channel masking may not be the best solution for this particular image.
But it's certainly not impossible, it's all in how you want to look at the problem.
I just copied the green channel.
Inverted it (Cnd/Cntrl + I)
Then I used the Brush Tool and the Polygonal Lasso Tool to outline the bag and fill in the BG of channel with black.
I then used a curves adjustment layer to bring up the white a little more.
Then I again used the Brush Tool (set to white) to fill in the bag.
Make a selection of the channel. Reset the RGB of the Channels Panel.
Go back to layers.
Highlight the Golf bag image.
Add layer mask
Now I can select the layer mask, right click and choose "refine mask" and make any fine adjustments it requires.