Game developers don't have the freedom that game modders do. There's a lot of behind-the-scenes approval processes that need to be coordinated by the team, schedules to be met, and developers often have to request higher ups to grant them more time on a project which often gets rejected because to the higher ups, meeting the deadline is more important than something finished. Something like My Wedding Stories was clearly rushed out, and it's very likely playtesters for that already played the pack and reported on the bugs we are already aware of (it's not like those bugs are very well-hidden, anyone who has played the pack can immediately tell it's in a slipshod state), but the higher ups didn't grant extra time to iron out the flaws, so the pack gets shipped out in a buggy, broken state. This is also a reason they can fix the bugs extremely quickly when the problems get serious enough, such as the aging bug.
Modders don't need to work on a schedule, they don't need a budget, and don't need to request upper management to grant them extensions. That's the single biggest advantage they have compared to paid professionals. While they don't have access to the source code and need to painstakingly tear the game apart to learn about it, they can still know enough about it to fix many oversights the game has, plus more.