I’m not a scientist, but I think the question you’re asking is the question they’re seeking to answer by searching for life in these places. There isn’t an assumption that if it had liquid water it may have had life (that statement alone inherently isn’t an assumption), they are searching for any evidence of life on Mars because A) it’s the closest and easiest planet to get to and B) while we know in what conditions life on Earth arose, we can’t be sure the same conditions are necessary somewhere else, for life that possibly may not be carbon based.
I’m not a scientist, but I think the question you’re asking is the question they’re seeking to answer by searching for life in these places. There isn’t an assumption that if it had liquid water it may have had life (that statement alone inherently isn’t an assumption), they are searching for any evidence of life on Mars because A) it’s the closest and easiest planet to get to and B) while we know in what conditions life on Earth arose, we can’t be sure the same conditions are necessary somewhere else, for life that possibly may not be carbon based.