Why do lets players often dub non voiced games?

[Something Amyss]

Fun, both for us and them, seems like a good reason.

 

[Johnny Novgorod]

It can be fun. The Grumps' dubbing of Trauma Center is so brilliant I wouldn't want to get the game if it didn't come with their commentary track included.

 

[CaitSeith]

I know its their job but can lets players just shut their yapper at certain times? Or at least explain why they chose to try and be voice actors?

Just showing walkthroughs doesn't make you memorable, and Youtube's Let's Play are about getting attention. Most LPs are about the Youtuber showing the game in an entertaining way. So, voicing dialog is a way to be entertaining in moments where there is no gameplay or nothing impressive is happening.

 

[Smooth Operator]

Well the alternative is dead air... that makes for a very poor show, their job after all is to entertain and it is done poorly if you have huge gaps of silence.
I do fully understand the problem, but you are asking it to be fixed by people who are doing a different kind of job.