Because you're a functioning member of society, that's why.
Or, more specifically, because you need to hack away at your workload.
Or, you know, just because it's Monday.
But seriously, have you considered taking a break and going for a walk outside? The fresh air will do you good, and you can still come back to your desk and hack away.
Or, if you're feeling particularly adventurous, you can try our sister page, Why Not Hack?.
Or, if you're feeling really stuck, check out our troubleshooting page: Troubleshooting: Because You Clearly Needed Help.
Or, if you're just plain bored, try our "Things to Do on a Monday" page: Things to Do on a Monday.