Why the heck do people sign off letters with best?

Because they are too lazy to type the remaining letters for whatever they are saying "best" to.

Best...what?

Best fishing?

Best Apples in New Zealand?

Best of luck in the heart surgery you weren't expecting you were going to need?

It's like signing off with "Since" instead of "Sincerely". Why not just sign off with out anything. It's about as meaningful. If you can't tell, this is a pet peeve of mine,

Since,

Me

(See what I mean?)