How do you know this? You recognize they probably did find and fix at least some bugs, oversights, and overtunes right? So they had some rate of fixes already. You know that finding a bug isn't the same as fixing it right? Developer time costs a lot and is in pretty short supply if you want them to be good. Fixing a bug as soon as you find it is almost never the most efficient way to handle things. Developers aren't just sitting around with nothing to do which means they can't just jump on to whatever latest issue is raised without disrupting what they were already working on.
No, instead when a bug gets raised either internally or reported by customers it gets roughly investigated, logged in a bug tracking system, assigned a team, assigned a priority, and only if it's enough of a show stopper to critically break things will it be started on that day. During normal cadence, any non-critical bug will at earliest not be picked up until the next "sprint", or repeated 2 week session (assuming common Agile practices), so it normally sits a number of days at least before anyone looks at it again because once again, there's only so many devs and they're already working on something.
Once the code has been changed then you get to go through testing, validation, building (packaging the code together into a releasable format), and distribution. Some of that is automated based on the company, but "automated" still doesn't mean it all happens in the flash of an eye. Your simple fix that you add to the codebase can end up clashing with another simple fix that someone else adds to the codebase on the same day and set one or both of you back some time.
Considering how lengthy and expensive of a process it ultimately is, the fact that ANet has been pushing out patches pretty frequently since EoD launch makes me think they had a long list of bugs already logged in their system ready to go on release date. So they had bugs to work on, they were working on bugs, they have been pushing out fixes for bugs, how can you actually know they would have done so any faster if they had done things differently?