Explicit variable declaration is just adding a keyword (such as var or let) when you're declaring a new variable instead of modifying one.

The cognitive burden of having to memorize and look for which variables are new vs which are being modified is simply not worth it in my opinion, even for a scripting language. Maybe for esolangs, simple math or first time learning programming.

In any case, it's a short coming of the language (IMO) but not a deal breaker. We learn to live with it.