Because meta is, in and of itself, a rather undisciplined proposition. It is in fact the antithesis of the high signal to noise environment that we strive to create on our main sites. That's precisely why we didn't have meta for quite a while.
It's quite polarizing, if you think about it - going from no discussion and no chit-chat to all discussion and all chit-chat. Then consider how voting is completely different on meta sites. We basically turn our engine on its head.
When participating in a community entails frequent journeys between these strangely parallel universes, a little fun and humor helps human beings take it all in stride. Tongues go into cheeks, while unicorns and one-armed dragons make much more frequent cameo appearances. I'll admit that initially we went a little overboard with shenanigans, but we've come around to a sort of norm where we get stuff done, while still taking advantage of being able to enjoy the company of one another just a little more than we otherwise could.
We're building cites on meta sites, collectively, and doing a pretty good job of it. To that extent, we're quite professional, in that we tend to consistently produce awesome things.
Since a little fun now and then doesn't really detract from that goal, we're more than happy to have a helping or two on occasion. This is particularly noticeable during the holidays when even employees loosen up a bit and reward people for simply enjoying the culture they created.
Think of Meta like our cafeteria, where the occasional food fight is generally tolerated, provided that everyone is having a good time and getting something constructive done in the process.