Because if shit blows up, there is time before the weekend to fix it.
No developer wants their game, especially a multi-player to be off-line on the weekend.
Most developer's bean-counters don't like paying OT for when shit breaks and they have to work the weekend/late into the night to fix it.
When you work late/long hours/weekends to fix major blocking issues (game crash/exploit/etc) targeted, but less thorough testing occurs.
At the latest, Thursday is the last day most developers like to drop an update/patch. I've seen it done on Fridays, and I've seen it go horribly wrong.