Lithium 9V Battery for Gun Safe

I'm reading mixed thoughts on using lithium (non rechargeable) 9V batteries for gun safes. I've been using one on my vaultek lifepod 2.0 with no issues for about a year but I recently bought a new safe from secureit and it specially says do not use li-ion batteries. Is there any reason why? A quick google search says it can be a fire hazard, then why are they even made? Sorry if this is a stupid question.