Here are several reasons why thunder and lightning should be synchronized:

- comments show that a lack of synchronization can be distracting
- it's cheaper/easier
- they're often synchronized in real life
- this is a show, which presumes a fiction

That first reason is the most important; the others are just to help technical perfectionists overcome themselves (you know who you are). As a distraction, the lack of synchronization is an enemy of your storytelling, and should be (mercilessly) edited out of the production.

Take it from a recovering technical perfectionist: accuracy is rarely required or beneficial to the storytelling (not to mention one's bank account.)