Because thanking someone means you're grateful for something they did or that you'll hope they'll do. It doesn't really apply to every email.

In contrast, expressing best wishes always makes sense unless you hate the recipient's guts and want them to know it.

Thanks, civilized

(See? It makes no sense.)

Every message you send is implicitly requesting something though - the time and attention of the person reading it.

Time is one of the most precious commodities we have - it seems appropriate to thank people for spending it on you.

Thanks, variaga

One thing is clear to me, everyone signing off on their comments is kinda nice.

Yours truly, DG

Agree, it brings a proper closing to the individual discourse.

Regards, The Lannisters