I know precisely why; if I choose not to reveal it here it may be because it's none of your business! I could have said that initially but that would have been rude not that rude seems to bother you!
That said, there is some merit in the idea of using a CC and covering the amount of purchase online and I will consider it.
One other thing, rarely do I need to enter more than the amount of my check by the time the checker finishes with my groceries. In other words, it takes no more time than it does to sign a credit card receipt which must be printed first and often takes more time than if I had just given a check. No one groans or is otherwise inconvenienced by my check writing because by the time my groceries have been scanned, my check is already finished. Even in the rare instances when that is not the case, the bagger is still bagging.
Further, if people come to the line with only a few items, I usually let them go ahead of me knowing I have a check to write. If I come to a checkout stand with only a few items and the next person offers me the chance to take their spot, if I have to write a check, I'll decline the offer, thanking them and informing them I have to write a check.
I'm not inconsiderate. I know precisely how it feels to be standing in line waiting for some dawdler at the front of the line to fumble around and/or spend a lot of time going over their coupons with the checker. Knowing that and how it makes me feel, I'm quite conscious of the feelings of others waiting in line and do the best I can to speed up the process.